Wednesday, 23 August 2017

A History of Leather



Primitive people, who lived during the Ice Age some 500,000 years ago, were likely the first to use the skins of animals to protect their bodies from the elements. Just as leather today is a by-product, our ancient ancestors hunted animals primarily for food, but once they had eaten the meat, they would clean the skin by scraping off the flesh and then sling it over their shoulders as a crude form of a coat. They also made footwear to protect their bare feet from rocks and thorns by taking smaller pieces of animal skin made to fit loosely over the foot and tied at the ankle with thin strips of skin or even vines.

The main problem that primitive man encountered was that after a relatively short time the skins decayed and rotted away. With his limited knowledge and experience, primitive man had no idea how to preserve these hides. As centuries passed it was noticed that several things could slow down the decay of leather. If the skins were stretched out and allowed to dry in the sun, it made them stiff and hard but they lasted much longer.

Various oily substances were then rubbed into the skins to soften them. As time passed, it was eventually discovered that the bark of certain trees contained "tannin" or tannic acid which could be used to convert raw skins into what we recognize today as leather. It is quite hard to substantiate chronologically at exactly what time this tanning method materialized, but the famous "Iceman" dating from at least 5,000 BC discovered in the Italian Alps several years ago, was clothed in very durable leather.

In recorded history, pieces of leather dating from 1300 B.C. have been found in Egypt. Primitive societies in Europe, Asia and North America all developed the technique of turning skins into leather goods independently of one another. The Greeks were using leather garments in the age of the Homeric heroes (circa 1200 B.C.) and the use of leather later spread throughout the Roman Empire. During the Middle Ages, the Chinese knew the art of making leather.

North American Indians - also had developed great skills in leather work, they took the ashes from their campfires, put water on them and soaked the skins in this solution. In a few weeks the hair and bits of flesh came off, leaving only the raw hide. This tanning method, which used a solution of hemlock and oak bark, took about three months to complete after which the leather was worked by hand to make the hide soft and pliable.

The Making of Leather
The tanning of leather was used by mankind in numerous geographical areas throughout the early periods of human civilization; the first rudimental tanning process is mentioned in Assyrian texts and in Homers Iliad. As certain leather characteristics began to emerge, men realized leather could be used for many purposes besides footwear and clothing. The uses and importance of leather increased greatly. For example, it was discovered that water would keep fresh and cool in a leather bag. It was also found suitable for such other items as tents, beds, rugs, carpet, armour and harnesses.

An early Nubian predynastic grave has revealed a leather vessel at the head of the occupant where a pottery one would normally be expected.

Ancient Egypt - one of the most developed civilizations in this early period, valued leather as an important item of trade. The Egyptians made leather, the historian, Strabo, tells of an interesting use developed by Phoenicians who made water pipes from it. They also made sandals, belts, bags, shields, harness, cushions and chair seats from tanned skins. Many of these items are in fact still made from leather today.

  The Hittites - one of the oldest civilizations in Anatolia, which is known as the leather production centre since the very old times, developed the art of tannery with aluminium during their civilization's brightest period between the years 2000-1200 B.C. These lands were rich in aluminium compounds and vegetal dressing pelts, and that made it possible for the tannery process to be completed under perfect conditions. During the excavations in Bogazkoy and Alisar, leather pieces were found in a boy's grave belongs to year 2800 B.C. The Hittites used gallnut and alum as dressing pelts in leather works.

              Greeks and Romans - used leather to make many different styles of sandals, boots and shoes, when the Roman legions marched in conquest across Europe, they were well attired in leather by wetting the leather In hot water, it will shrink drastically and partly gelatinize, becoming rigid and eventually brittle. Boiled leather is an example of this, where the leather has been hardened by being immersed in hot water, or in boiled wax or similar substances. Historically, it was occasionally used as armour after hardening, the shield carried by the ordinary soldier was more likely to be made of leather than metal and it has also been used for bookbinding.

The ancient Greeks refer to eight basic guilds of artisans, which included both shoemakers and tanners. Although tanning was originally a cottage trade, the Greeks had full-time professional tanners who were at first employed in leather processing establishments and became independent some time later. The barks of conifers and alder were used as tannin sources and so were the peel of the pomegranate, sumach leaves, walnut, cups of acorns as well as an Egyptian heritage - mimosa bark.

The Greeks were also familiar with alum tanning and it appears they knew something about tanning with fish oil. The types of leathers used were as diversified as the end users. Homer refers to the use of cowhide, goat and weasel leather by the Greeks.
A tannery was uncovered amid the ruins of Pompeii and the same equipment of the kind still in use for centuries thereafter was found in it. The edict issued by the Roman emperor Diocletian which fixed ceiling prices for all kinds of goods and services included skins and leather prepared from goats, sheep, lambs, hyenas, deer, wild sheep, wolves, martens, beaver, bears, jackals, seals, leopards and lions. Under the edict, cowhide was even classified according to groups and qualities. A complete tannery in the famous ash-preserved ruins of Pompeii was unearthed in 1873.

Middle Ages,
As we move into the middle ages, leather continued to increase in popularity. By far the cleverest craftsmen with leather in medieval times were the Arabs. The Moors developed remarkable skill primarily in the preparation of beautiful goatskin still known as morocco leather after the country of its origin. In fact the description 'genuine morocco' is still very highly regarded today, particularly in the manufacture of small leather goods.

Medieval England - ancient Britons had many uses for leather from footwear, clothing and leather bags, to articles of warfare. The hulls of the early boats, known as coracles, were also covered in leather. Through the centuries leather manufacture expanded steadily and by mediaeval times most towns and villages had a tannery, situated on the local stream or river, which they used as a source of water for processing and as a source of power for their water wheel driven machines

All kinds of containers were made from leather, such as sword cases and dagger sheaths, box coverings and water bottles, many of them beautifully decorated by punching and incising. Leather was also a favourite medium for decorative art. Leather was used to cover books. In those days, when the horse was the principal means of transport, saddler and harness making were important uses of leather.

Britain has been the home of leather vessels for longer and in higher numbers than anywhere else in history and their existence has become quintessentially British.

The Black Jack`s name is derived from the materials used in its construction. Leather that has been soaked in hot water and dried is known as Jack leather. The same source can be attributed to the name for German Jackboots and Medieval Arming Jacks. This is also the origin of the modern word “jacket“. Jacks were originally black because the black material used to line the inside, was used on the outside of the vessel thus colouring it.

In the early 1900s, the brown leather flight jackets worn by aviators and members of the military, commonly called "bomber jackets", were prized for their comfort and durability. The jacket was often part of an overall uniform ensemble meant to protect fliers from exposure to the extreme climate conditions found at high altitude, and sometimes incorporated sheepskin, using the intact fleece on the inside for warmth.

Modern Day
Until the later part of the 19th century, there were relatively few changes in the methods used to produce leather. In fact, the process had changed very little in over 200 years. However, the industrial revolution did not bypass tanning - one of the oldest and most basic forms of manufacturing. Science was quickly introduced to the art and craft of leather making. A wider range of dyestuffs, synthetic tanning agents and oils were introduced. Together with precision machinery, these changes and continued innovations to the present day have combined to make tanning into a viable, modern manufacturing industry.

Associated Articles


The Physics of Polishing -



Physics (from Ancient Greek: φυσική (ἐπιστήμη) phusikḗ (epistḗmē) "knowledge of nature", from φύσις phúsis"nature"

 [: a natural science that involves the study of matter and its motion through space and time, along with related concepts such as energy and force.]

The clear coat will generally measure between 1.5 - 2.5 mils. The clear coat provides gloss plus physical protection from the elements, including ultraviolet (UV) radiation, which is in the upper level of a cured clear coat. 

Most car manufacturers will only allow ~ 25% of the clear-coat thickness to be removed without voiding the paint warranty and long-term durability problems becoming an issue. That means that if you started off with 50µ of clear coat (this will vary by vehicle manufacturer) you would only be able to remove <12µ without voiding the paint warranty and possibly having a re-paint.

New or Aged Paint
 Be cognizant there are major differences between the way a freshly painted and an aged surface will react to abrasives, which are usually aluminium oxide or silicon carbide Abrasive products are designed for newly painted surfaces and for use by body shops not  paint correction/renovation.  Both types of paint require a different approach and perhaps even a different abrasive product. 

It is all too easy to remove too much paint once it is ‘aged’ with the inherent danger of removing it’s ultra violet protection. Paint surface polishing limits - 6.26 mµ (0.25 Mil) < clear coat removal. Any more then this will result in paint failure and a re-paint. Most compound type abrasives are formulated to remove sanding marks from fresh paint not paint renovation

Pad Brasivness

Pads have a grit number, which is in effect how abrasive it is (this grit number is based upon how much grit is contained in a sqare ince of finishing (sandpaper) paper. So an abrasivene can be affected by the grit number of the pad.The grit size of sandpaper is usually stated as a number that is inversely related to the particle size. A small number such as 20 or 40 indicates a coarse grit, while a large number such as 1500 indicates a fine grit.

Polishing Abrasives

I’ve spent many hours meeting and speaking with Engineers, paint chemists, product formulators, and abrasive manufacturers and polishing pad manufacturers and have gained insights into paint surface polishing that has afforded me an understanding of how all these things interact to form a process, on a scientific level paint polishing is the sum of all these parts.
Polish a paint surface correctly (regardless of the machine we are using) the goal is leave a series of scratching that is so fine that it becomes imperceptible to the naked eye.

Backing Plate / Pad Motion

Inertial mass [: is mainly defined by Newton's law, the all-too-famous F = ma, which states that when a force F is applied to an object, it will accelerate proportionally, and that constant of proportion is the mass of that object]

To determine the inertial mass, you apply a force of F Newton’s to an object, measure the acceleration in m/s2, and F/a will give you the inertial mass m in kilograms.
Centripetal and Centrifugal force should not be confused with each other, they are opposites

Centripetal [: is a force which acts on a body moving in a circular path and is directed towards the centre around which the body is moving]

Centrifugal [: a force, equal and opposite to the centripetal force, drawing a rotating body away from the center of rotation, caused by the inertia of the body]

A vibration free tool will fatigue the user less and can reduce the risk of long-term health problems such as carpel tunnel syndrome (see article “Health Hazards of Detailing”) and it also reduces most of the factors that cause wear to the motor.

Random orbital polishers, by design, are unbalanced. You have a large mass (backing plate/pad/spindle) that is orbiting around a central axis point. The further the mass is away from the axis, the greater the unbalance, the more intense the vibration becomes (see diagram Centripetal Force)
Dynamic balance [: occurs when the force on all sizes of the axis are equal and then the tool operates with very little ‘out of balance’ vibration] the uneven distribution of mass in a rotating body contributes to the unbalance.

Centre of gravity [: a point from which the weight of a body or system may be considered to act. In uniform gravity it is the same as the centre of mass.

Excessive vibration in rotating machinery can cause unacceptable levels of noise and substantially reduce the life of shaft bearings. Hence, the ideal would be to remove all causes of vibration and run the unit totally smooth. Unfortunately, in actual practice, this ideal cannot be achieved and some inherent cause of vibration, or unbalance, will remain.

The best you can do is to reduce this unbalance to a level that will not adversely affect the bearing life and will reduce noise levels to an acceptable level. The unbalance is caused by an effective displacement of the mass centre line from the true axis caused by some mass eccentricity in the unit.

The benefits of a large orbit or elliptical offset (15-21 mm) is that more centripetal force is created as the backing plate orbits, so the backing plate will rotate more than a similar machine featuring a smaller elliptical offset. A machine featuring a large stroke delivers increased speed of backing plate motion using the same orbits per minute.

A large stroke elliptical offset increases movement of the baking plate / pad, which increases the speed of the backing because they are travelling a greater distance in the same amount of time. The resultant speed also increases the development of kinetic energy, therefore levelling is more consistent.

This type of movement helps to remove residue (oxidized paint, spent abrasives, etc.) more readily than a small elliptical offset and extends the workable life of the pad. This culminates in a very smooth polishing action; making it extremely comfortable to use even when working for prolonged periods.

A majority of random orbital machines use an elliptical offset between 3.0-8.0 mm. It is generally accepted that a smaller stroke leaves a more refined finish, but experience shows that this type of movement doesn’t readily clear (oxidized paint, spent abrasives, etc. ) polishing debris, thereby blocking the pores of the pad and placing debris between the pad and the paints surface negatively impacting the abrasive backing plate / pad motion

Why Low Vibration is Important
Vibration can cause a range of conditions called hand-arm vibration syndrome (HAVS). The best known is vibration white finger (VWF), but vibration also links to specific diseases such as carpal tunnel syndrome.

HAVS and VWF usually result from damage to the fine capillaries in the end of your fingers which reduces blood flow to the extremities of fingers and hands which take on a 'blanched' effect appearing white.

It has been proven that operators face significant risk of muscle, joint and nerve damage caused by the inherent vibration of power tools. By reducing the amount of vibration you can reduce muscle and joint fatigue as well as the potential risk for permanent damage, as described in ISO 5349-1:2001- Mechanical vibration -Measurement and evaluation of human exposure to hand-transmitted vibration.

Declaration of Vibration Emission
The Supply of Machinery (Safety) Regulations require that, among other information, suppliers of machinery must declare the vibration emission of their tools and machines. The purpose of declaring such information is to allow purchasers and users of tools and machinery to make informed choices regarding the vibration emission of a potential purchase

The method of declaring vibration emission is to apply a standard test to a machine or tool. The purpose of the standard test is to provide a repeatable and reproducible method of estimating vibration emission. This will bring the process of emission declaration in line with the techniques for assessment of human exposure to hand transmitted vibration as defined in EN ISO 5349:2001 and as called for in the requirements for quantification of exposure in the Control of Vibration at Work Regulations (2005).

Surface Lubrication

Glycerin (C3H8O3) [: is a simple polyol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in all lipids known as triglycerides.It’s trihydric sugar, being the alcoholic component of fats ]

Surface (Contact) Area
The distance around the circle is a circumference (c) The distance across the circle is the diameter (d). The radius (r) is the distance from the centre to a point on the circle. (π= 3.14), d = 2(r), c = d = 2 (p) r, A = π (r) 2 Even a minor change in pad diameter makes a big difference in surface area.

A pad should be designed to efficiently use its surface area. Foam pads that have lines, squares, circles, or dimples cut out of the pad face, means there is less actual surface area in contact with the paint surface. Area = π (r2) 6-inch pad area = 18.842 sq.ins, a 4-inch pad 12.46 sq.ins.

Kinetic (Heat) and machine energy (Speed) and surface pressure applied over a smaller area, which results in faster correction. A further consideration of pad diameter has to do with distribution of the machine weight and applied pressure.

Another design parameter that determines how much surface area actually contacts the paint when using foam pads is the amount of pores per inch it features (commonly referred to as PPI). More pores, larger pores, thinner walls between the pores, or how stiff the walls are all affect how much foam contacts the paint during the buffing process

Block wet sanding (finishing paper and a sanding block) which ensures a consistent pressure over the surface contact area, this is the most effective tool for paint defect removal because of its linear process you abrade the paint surface flat until the defects are removed.

Pad Velocity (Speed)
The larger the pad, the greater the pad velocity at an identical RPM; V = RPM (Area) A - 6.5-inch = 20.423 sq.ins V= 24,507 inches per minute (IPM) Pad velocity is also substantially increased with a larger diameter pad, which increase its abrasive ability at the outer edge; 8-inch =25.136 sq.ins. V= 30,163 RPM

Pressure / Pad Compression
Depending on the types of surface abrasions you're dealing with, increase pressure is necessary; otherwise most of the kinetic energy of the machine will be absorbed by the pad (especially foam) and not transferred to the paint surface.

Just remember that more pressure equals more aggressive, so be careful around ridges and raised surfaces Maintain the same pressure and work the product in, it may take three or four passes to complete before the residue can be removed. Once you see the desired results move on to the next area, or repeat the process as necessary.

The required pressure applied to obtain optimum results to adequately compress the pad (50%) and obtain uniform abrasion is usually in the range of 10 to 15 lbs. (a random orbital buffer will stall at approximately 20 pounds of applied force) use just enough pressure to keep the pad rotating at 1-2 rotations per second.

To compress a 6-inch pad 50% requires you increase the total force by the ratio of its surface areas; Ratio = [Ï€ (radius2)] / [Ï€ (radius2)] = 2.25 as much force, almost 34 pounds).

With the smaller pad you're applying the same force, at a constant speed but over a smaller, more concentrated area, which will induce friction and greater abrasion abilities to the polish, both these abilities require a certain amount of caution as it’s possible to abrasion burn the paint.

Constant Pressure -  Foam Pads have a layer of engineered, instant rebound foam between the pad and the backing plate. This layer acts as a cushion or shock absorber between the machine, the operator and the surface being worked on. It absorbs off-axis motion while maintaining a constant and uniform pressure on the surface; Lake Country Mfg. Constant Pressure technology allows even a neophyte detailer to achieve professional-like results.

Foam Pad Size (Area and Applied Pressure)
Different pad sizes can have an impact on how the buffer breaks down a polish, as it applies its dynamic friction over less area, control, better manoeuvrability, and how fast you can cover an area.
Smaller pads in general will offer you more control with any machine polisher, as it can reduce the tendency for the buffer to hop or skip on the paint. Smaller pads also make it easier to manoeuvre buffers in tighter areas and closer to trim pieces.

The low profile 5.5-inch buffing pads pack the same CCS technology and performance into a compact, highly effective size that works best with dual action polishers and air sanders. Use with a 5 - inch moulded urethane backing plate for excellent flexibility and balance by Lake Country (LC) manufacturing

Assuming equal speed, radius and foam compression (50% - 15 pounds of force applied) the difference between 4- inch and 6 - inch pads is their different surface area = π (r2) (4-inch = 12.46 sq.ins / 6-inch = 28.26 sq.ins) and therefore surface kinetic (or dynamic) friction applied and surface pressure applied 4-inch = 3.75 lbs per sq.ins. - 6-inch = 2.5 lbs per sq.ins. Even a minor change in pad diameter makes a big difference in surface area.

Actual Speed (RPM)

Formula - RPM (C) = V
Speed = Revolutions per minute, C = circumference (2(π) (r)), V = velocity
Area 7.5 -inch at 1,800 rpm (usable area 6- inches)
Speed - 1,800 (rpm) (6" diameter times pi = 6 -inch x 3.14) x 18.84 = 33,912.
So, using the same formula, and a problem of: X (unknown rpm) x 25.12 (8" diameter times pi = 8 x 3.14) = 33,912) then; 33,912 / 25.12 = 1,350 actual speed rpm

Heat from Kinetic (or dynamic) Friction

[Energy in a system may take on various forms (e.g. kinetic, potential, heat, light). Kinetic friction, or surface resistance induced heat; an often misunderstood concept of polishing / compounding; abrasives require friction to breakdown, not heat; heat is just a resultant of friction between two surfaces. Kinetic friction is required to ‘level’ paint, which is simply the removal of paint to the lowest point of the paint defect] 

Polishing a paint surfaces transfer’s kinetic friction induced heat to the paint surface, thermoplastic polymers have both tensile strength (a linear stress-strain relationship) and elongation (elasticity) which allow the surface to flex, expand and contract in accordance to surrounding temperatures, solvents, resins and other ingredients in polishes will expand causing the paint film surface to expand

As the metal substrate expands the paint moves with it, due to its elasticity, thereby becoming elongated (thinner) this is part of the cause of friction induced ‘burn’, you’re applying pressure and an abrasive to a less dense (‘thinner’) paint surface, excess friction induced heat can cause the paint surface to burn, blister, haze, and cause excessive swirls.

Plastic has a much lower rate of thermal conductivity than metal, so it absorbs heats at a far greater rate and therefore is subject to localized ‘spot’ heating.

Polishes and compounds do not need heat per se for the abrasives to polish a surface, whether they be diminishing or non-diminishing abrasive, they require both pressure and friction Kinetic friction induced heat can cause a rapid temperature rise; (i.e. initial surface temp 80.F, friction heat attained with the polisher stationary and a cutting foam pad at 1,100 RPM for approx. ten seconds the friction induced heat attained would be around ~104.F)

When a localized spot is significantly hotter that the surrounding area you have a potential problem, the paint temperature can be checked by utilizing an instant read-out infrared ‘gun type’ digital thermometer

A finishing pad will not provide as much friction as a cutting foam pad (less surface resistance) although they will both produce friction induced heat, whereas a wool pad, due to their composition, creates less friction induced heat but more kinetic friction (due to its fibrous structure) than most foam pads. Be cognizant that with high ‘spot’ temperatures the pad structure will cause scratching that is forced deep into the clear coat.

You should try to maintain as cool a surface temperature as possible when polishing, use your hand to provide a general idea of how much heat is being transferred by kinetic (friction) energy; it will of course be at a higher temperature than ambient. An orbital polisher tends to concentrate higher temperatures at the centre of the pad, whereas a rotary the heat builds up more toward the outer edge of the pad.

Kinetic Friction induced heat can cause a rapid temperature rise; (i.e. initial surface temp 80.oF, friction heat attained with the polisher stationary and a cutting foam pad at 1,100 RPM for approx. ten seconds the friction induced heat attained would be around 104.oF) the paint temperature can be checked by utilizing an instant read-out infra-red ‘gun type’ digital thermometer, paint surface ‘spot’ temperature should be limited to 110.oF <

In accordance with the Society of Automotive Engineers (SAE) a localized (spot) temperature of > 115.oF will cause the urethane clear coat to soften and the foam pad will cause scratching that is forced deep into the clear coat. (See also the first law of thermodynamics et al)

The result of creating heat in an OEM clear in excess of 145 F, creates thermo-stressing of the resin system and as a result the clear coat would suffer premature failure in a short amount of it's life.

Polishing Freshly Applied Paint
When a urethane clear coat is sprayed its outermost surface, measuring a few nanometres in thickness, sustains microscopic fractures when it comes into contact with air. These fractures are microns or nanometres in width and thus too small to be seen with the unaided eye. 

Freshly applied paint that in the outgas stage, is still full of evaporating solvents, and is usually less dense (soft) despite the additives used (hardener) once a catalyst, kinetic energy (friction heat) is added, it causes the paint film to expand, temporarily hiding scratches, this is often the reason for a body-shops bad reputation of returning vehicles that have sanding scratches in newly applied paint that should have been removed.

Be cognizant when polishing newly applied paint the kinetic energy (heat) from a foam pad can also cause solvent engorgement, which causes the paint film to thin due to the expansion of the evaporating gases, applied rotational force may also cause the paint to tear Kinetic friction (heat) is transferred to a solvent (IPA or fresh paint solvents) causing it to both expand (Charles' law; also known as the law of volumes) the paint film and soften it.

Automotive paint is classified as a semi-permeable membrane; it has both tensile strength and elongation (elasticity) newly painted surfaces are soft and full of out gassing solvents, resin binders and additives, as well as and water.

Polish contains solvents, which soften the paint film, kinetic surface friction and applied downward pressure transfers its energy into heat / torque (force to rotate an object about an axis), which could result in the alteration of the paint films bond between its substrate, causing it to delaminate or tear?



The heat makes the gasses expand (pV = nRT) the expanding gases go through a phase transition (change in density) and to relive this increased pressure they (a) rupture the paint film surface, causing small fissures (similar to solvent pop) The heat may cause the gaseous vapours to expand, but not enough to break through the hardening clear coat. Once the vapour has evaporated, it may leave a void between the basecoat and the clear. Therefore, you have a cloudy spot where the clear and base is no longer adhered together. If this is the case, the clear coat will delaminate in the future.

Once the outgas process is complete automotive coatings (paint) becomes a semi-solid permeable membrane, being a polymer (elastomers) it remains flexible while retaining its tensile strength, to enable it to expand and contract to follow temperature fluctuations of the substrate (elongation). 

Kinetic friction and its associated heat can cause a rapid temperature rise (i.e. initial surface temp 80.oF, heat attained with a cutting foam pad at 1,100 RPM for approx. ten seconds is approx. 104.oF) the paint temperature can be checked by utilizing an instant read-out infrared  thermometer, paint surface spot’ temperature should be limited to 110.oF <



Force diagram
Arrows are vectors indicating directions and magnitudes of forces. W is the force of weight, N is the normal force, F is an applied force of unidentified type, and Ff is the force of kinetic friction which is equal to the coefficient of kinetic friction times the normal force. Since the magnitude of the applied force is greater than the magnitude of the force of kinetic friction opposing it, the block is moving to the left.

Applied Pressure
The pad needs to have an even distribution of pressure applied to it; depending on the types of surface abrasions you're dealing with, increase pressure as necessary. Just remember that more pressure equals more aggressive, so be careful around ridges and raised surfaces
Maintain the same pressure and work the product in, it may take three or four passes to complete before the residue can be removed. Once you see the desired results move on to the next area, or repeat the process as necessary.
The required pressure applied to obtain optimum results to adequately compress the pad (50%) and obtain uniform abrasion is usually in the range of 10 - 15 lbs. (a random orbital buffer will stall at approximately 20 pounds of applied force) To compress a 6-inch pad 50% requires you increase the total force by the ratio of its surface areas

Ratio = [Ï (radius2)] / [Ï (radius2)] = 2.25 as much force, almost 34 psi). With the smaller pad you're applying the same force, at a constant speed but over a smaller, more concentrated area, which will induce an increase in friction and greater abrasion abilities to the polish / pad combination, both these abilities require a certain amount of caution as it’s possible to ‘strike through’ (friction burn) the paint.

With the smaller pad you're applying the same force, at a constant speed but over a smaller, more concentrated area, which will induce an increase in friction and greater abrasion abilities to the polish / pad combination, both these abilities require a certain amount of caution as it’s possible to ‘strike through’ (friction burn) the paint.

Hardness
So how can a dense (hard) clear coat be so easily scratched?
It’s a matter of physics, not material density (material hardness). Force acts through a body that has a surface area; if the surface area is really small while maintaining an equal force, the pressure becomes astronomical and the object under pressure capable of penetrating the surface of an otherwise tough material.

Newton's third law of motion [: when a first body exerts a force F1 on a second body, the second body simultaneously exerts a force F2 = −F1 on the first body. This means that F1 and F2 are equal in magnitude and opposite in direction]

That’s why a micro fine thread that is twice as fine as silk and a 100 times finer than a human hair, in an otherwise soft towel will scratch your paint. And the same reason a mosquito can penetrate a rhino hide with its proboscis (stinger).

If you press down on your paint finish with your palm it feels really hard and tough, but that’s because the surface area of your palm is relatively large and what you’re actually feeling is the resistance of the steel underneath the paint. Try pressing your thumb nail into the paint with the same amount of force you used with your palm, if you dare.

Personal Protection Equipment (PPE)

1. Eye Protection: I would strongly advise the wearing of safety glasses or visor (prescription eyeglasses are not a substitute) when operating any machine polisher. OSHA requires employers to ensure the safety of all employees in the work environment. Eye and face protection must be provided whenever necessary to protect against chemical, environmental, radiological or mechanical irritants and hazards.
 2. Hearing Protection; the constant pitch of a polishing machine could affect your hearing so wearing ear plugs would be wise to protect you from hearing loss.
 3. Hand Protection; Gloves- with the verity of chemicals a detailer uses on a daily basis wearing chemical-resistant gloves resist penetration and permeation, and will provide protection against dermatitis and chemical burns. Gloves can provide protection, but they must be chosen with care, the proper selection matched to the hazard is critical as they offer a much needed protective barrier when handling cleaning chemicals such as wheel cleaners and multipurpose cleaners.
Nitrile gloves are made of synthetic latex. They contain no latex proteins and offer excellent resistance to punctures and tears. Nitrile gloves are three times more puncture resistant than rubber and can be used to offer superior resistance to many types of chemicals.
Chemical-resistant gloves resist penetration and permeation, and cam protect against dermatitis, chemical burns and corrosion. Nitrile gloves are three times more puncture resistant than latex rubber and can be used to offer superior resistance to many types of chemicals. Unlike other latex gloves, Nitrile gloves have low resistance to friction and are very easy to slide on –
Clove Chemical Resistance Chart - http://www.adenna.com/pdf/ChemicalsResistance.pdf
 4. Respiratory Protection (N95): Materials such as aluminium oxide (Aluminium oxide is on EPA's TRI list if it is a fibrous form) or silicon carbide (Nuisance particulate-Accumulation in lungs) used in polishes and compounds, and powdered fillers
Crystalline silica (polishes and compounds) poses a serious inhalation hazard because it can cause silicosis and Isocyanate clear coat residue represent a hazard to your lungs and may cause respiratory distress. Use a NIOSH-approved half face respirator equipped with a combination filter cartridge should be worn while using them

Consult the current 3M Respiratory Selection Guide for additional information or call 1-800-243-4630 for 3M technical assistance.

References
1. Correlation between vibration emission and vibration during real use - Polishers and sanders. Prepared by the Health and Safety Laboratory Health and Safety Executive 2007
Related Articles

Other Authors
1. Kevin Brown - http://www.buffdaddy.com/buffingpads

 I would like to think that these articles become an asset to anyone who is new to detailing and to professionals alike, as well as industry experts who seek to advance their knowledge.
I hope the above article was informative. By having some understanding of the ‘What’ and ‘Why’ as well as the ‘How’ along with a little science to help you understand how the chemicals we use react, you can achieve the results you desire.
I would appreciate it if you would share this article as it helps other detailers further their knowledge. Questions and/ or constructive comments are always appreciated.

Copyright © 2002 - 2012 TOGWT® (Established 1980) all rights reserved

Tuesday, 22 August 2017

Surface Hardness and Scratch Resistance



Surface Hardness

Definition [: the ability of a material to resist local deformation (or penetration) from externally applied pressure, and is directly related to its tensile strength; stronger materials are generally harder]

The enamel paint finishes on vehicles from the 50’s and 60’s era were as tough as porcelain. But rightly due to environmental concerns, those high percentage petroleum based paints have been generally superseded, resulting in the softer water-based paint finishes of today and the unavoidable orange-peel seen on many new and re-painted vehicles.

Today’s paints, unfortunately, rank somewhere near the bottom of the scale of hardness, especially single coat black/red paint the exception being a white single stage and CeramiClear, when compared to all the materials your paint can possibly come in contact with (always bear that in mind).
Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material

An adaptation of that hardness scale (1 - 10)
• Talc = 1
• Carbon Black [black paint pigmentation] = 2
• Glass = 6
• Titanium dioxide [white paint pigmentation] = 7
• Corundum 9
• Diamond =10

Pencil Hardness Test
This type of test uses special graphite pencils with different degrees of hardness to scratch the coating, which then determines its hardness - Testing Your Coating's Hardness
Pencil Hardness is one of many tests that are done to evaluate a coating's performance. Other tests are abrasion, reverse impact resistance, direct impact resistance, cross-hatch adhesion, oxidation, gloss retention, UV resistance, yellowing, blistering, drying times, chemical/solvent resistance (using both the rubbing and spot/time tests), salt spray resistance, humidity resistance, acid and caustic resistance
Most coatings are formulated for specific types of finishes, various conditions or different substrates. So use the pencil hardness test as one criterion for selection.

But do not judge any coating by coating thickness or pencil hardness alone, as there are many other significant characteristics to consider. Be cognizant that whatever the hardness rating it only offers scratch resistance, which only helps to minimize the appearance of light scratches

Pencil Hardness for Common Coatings
• Catalyzed polyester: 9H
• Catalyzed polyurethane: 9H
• Catalyzed modified acrylic polyurethane: 4H
• Catalyzed acrylic polyurethane: 2H
• Water-based polyurethane: 3H
• Water-based urethane/Isocyanate catalyst: 2H
• Urethane/nitrocellulose lacquer: F (24 hours)
• Water reducible lacquer: 2H
• Water-based polyurethane wipe-on finish: HB-F
• Clear shellac aerosol: 3B
• Polyurethane/nitrocellulose aerosol: HB
• Nitrocellulose aerosol: 3B

Hardness
Hard and soft are both relative terms; you can scratch the hard surface of vehicles paint with a soft towel by the application of enough pressure.

Both pressure and mechanical stress are defined as force per unit area. These two forces are the subject of Newton's third law of motion; the law of reciprocal actions [: to every action, there is an equal and opposite reaction] 

How can a hard clear coat be so easily scratched?
Force acts through a body that has a surface area; if the surface area is really small while maintaining an equal force, the pressure becomes astronomical and the object under pressure capable of penetrating the surface of an otherwise tough material.
That’s why a micro fine thread that is twice as fine as silk and 100 times finer than a human hair, in an otherwise soft towel will scratch your paint. And the same reason a mosquito can penetrate a rhino hide with its proboscis (stinger)

Scratch Resistance
Preserving a scratch-free, high gloss finish over a longer vehicle life has challenged the auto industry for decades. One problem is acid etch, degradation of the surface by environmental pollution (Acid rain, Industrial fallout, etc) a major factor in the phenomenon more generally known as weathering; another is scratch resistance to abrasion from many sources, not least of which is the car wash. 

Unfortunately, any solution comes with tradeoffs in paint chemistry. A scratch resistant coating was not as environmentally resistant, and vice-versa. And chemistries which offered the best combination were not VOC compliant (water- based) so applying them released solvent volatiles from the paint
Thickness of a paint or coating has little relevance; a coating will provide 2-4 µ (microns) a clear coat is approx 50-76µ and yet a micro fibre can cause scratches

Do not judge a coating solely by the thickness it provides or pencil hardness alone, as there are many other significant characteristics to consider. Be cognizant that whatever the hardness rating it only offers scratch resistance, which only helps to minimize the appearance of light scratches



Relevant Articles
1. American Society for Testing and Materials (ASTM) publications - http://www.astm.org/Standard/standards-and-publications.html

2. ASTM D1014 - 09 Standard Practice for Conducting Exterior Exposure Tests of Paints and Coatings on Metal Substrates
 3. Nanotechnology coatings “

Sunday, 20 August 2017

“Using Oil-based Leather Care Products”



Research
Information regarding the care of leather is scarce, often contradictory, misleading, or simply wrong. Misinformation can lead to inadvertent damage to your vehicles leather upholstery; my goal is to present clear, concise, accurate information. 

There is a great deal of conflicting information on leather care being put out by leather experts themselves who use baffling pseudo scientific techno speak as another marketing ploy, which makes it difficult to find a definitive, unbiased answer. It had always confounded me that such a simple subject has been made into something so complicated.

After various meetings and discussions with leather tanners, their chemists and fat liquoring formulators and many leather care product manufacturers I’ve gained an understanding of this versatile material on both a practical and scientific level.

I have always thought that the more facts and information you have at hand the easier it is to judge what information you are being given. After all, how can you fully understand and properly use any product unless you have all the facts? In the final analysis; it’s your vehicle, your hard earned money and your choice
Here is one definitive truth –you are dealing with the leathers finish, not the hide itself.
Materials Technology

Automotive OEM technology is becoming more and more complex requiring educated and skilled technicians to work on them. As the materials used are constantly changing we must maintain our knowledge base and utilize the correct products and application methodologies to keep up with emerging technologies.

Automobile model ranges use different materials for their vehicles interiors; leather upholstery like Aniline Immersion Dyed, Aniline Micro Pigmented, (Urethane) Finished, Artificial leather such as MB-Tex and unfinished materials like Synthetics and Alcantara, and sometimes combinations of products (Alcantara seat inserts on leather seating) as well as various grades of leather hide, full-grain, top-grain and split –grain (which is protected with urethane) all of which require different products and applications methods

Leather Tanning Process
All cowhides are naturally oily, unfortunately, these natural oils are stripped away in the tanning process, which is the process that renders the hide invulnerable to decay and some equivalent oils must be re-introduced after tanning. This last tanning step, the replacement of oils, is called "fat liquoring." Over the centuries, a number of oils have been found that have a natural affinity for leather fibres.

Every leather tanner has his own, unique, blend of tanning oils. These formulas are closely held secrets, passed down through generations; they are neither volatile nor migratory, this is the origin of the new car ‘leather smell’. This is one reason why one company's leather can have a totally different feel, fragrance, texture and softness from another company's product (See article “Fat Liquoring”)

Using Oil-based Leather Care Products
I get more questions on auto leather care than almost any other subject (glass cleaning comes a very close second) 

When it comes to vehicle leather upholstery care information, there are plenty of myths and very few real facts. I hope this extract will provide you with enough commercially unbiased, factual and relevant information to eradicate those myths to enable you to give your car's leather interior the proper care it needs to stay supple and looking great for many years.

Many of the following statements are controversial and are polar opposite of popular leather care practices. I've found that some leather care myths are deliberately perpetuated by the industry, especially those on the use of oil-based leather conditioners and others are just common errors of judgment.

The interior environment of an automobile can be extremely demanding on any material used. Temperatures range from hot dry summer days, to freezing nights. Both high and low humidity, even air conditioning that cools, but also dries. Leather's greatest enemies are; sun, heat, body oils, perspiration (that contains urea as well as organic salts and acids) and ultra violet radiation (UV), which dries the hide, fades the colour by bleaching, and can cause the leather to fail by drying out the fibres causing the urethane and / or the hide to crack.

When leather tanners talk about conditioning leather they are referring to re-hydration; not the replenishment or replacement of the fat liquoring oils and waxes. Modern finished leather needs to be kept hydrated with moisture to ensure the leather remains flexible and maintains its soft tactile feel.

This is done by regularly wiping the surface with a damp 100% cotton micro fibre towel and by using aqueous (water- based) leather care products. There is no reason to use oil-based leather care products to condition or feed leather hides

Automotive leather upholstery is a multi-strata urethane coating that allows hydration (transpiration and evaporation of moisture); consisting of the actual hide, colour pigmentation and the surface finish. An acrylic and polyurethane resin binder system is used to improve flexibility, fastness and adhesion to the leather, then two or three aqueous (water- based) pigmented base coat applications a clear aqueous (water- based) top coat is then applied as the final stage of the finishing process.  

Urethane and pigmented finished leather has micro-pores that allow evaporation and hydration (the passage of water vapour) they are not sealed per se as liquid vapours penetrate it easily; others liquids stay on top dependent upon their molecule size.

The urethane  used for protecting automobile upholstery is classified as a semi-solid permeable membrane, being a thermosetting polymer (elastomers) it remains flexible while retaining its tensile strength, to enable it to expand and contract, following the temperature fluctuations (elasticity) of the substrate.

The urethane although very resilient to abrasion wear from entering and exiting the vehicle, still maintains its physical properties like flexibility, tactile hand and its patina. A urethanes fibre structure will stretch in all directions with no particular grain or stress pattern. The urethane surface coating will  not withstand multi directional stress,  however, and when it’s flexed or stretched continuously in the same place the surface coating develops minute cracks. It also has micro-pores that allow evaporation and hydration (the passage of water vapour through a membrane or pore) they are not sealed per se as some liquids penetrate it easily; others stay on top dependent upon their molecule size

Modern automotive leather upholstery use a completely different tanning  processes and finishing system, utilizing advanced polymers and chemicals (urethane doesn’t require conditioning or rejuvenation) and as a consequence  they do not need to be treated with aftercare products containing oils or proteins.

Oil combines with little bits of dust and dirt, acting like a fine sand paper that wears down the protective coating on your seats as passengers get in and out of your car.  That protective layer makes your leather seats more resilient to scratches, water and heat damage as well as other types of wear and tear, so, once that layer is worn thin, your seats are more susceptible to all types of damage.

The percentage of oil on the weight of leather is quite small, from 3-10 %. The precise manner in which this small quantity of oil is distributed throughout the leather materially affects the subsequent finishing operations and the character of the leather.

Oils and soft plastics i.e. polymers, acrylics and urethanes are not compatible; repeated application on to finished leather can cause the breakdown of cross-linking and binding agents. 
Oil accelerates the deterioration of urethane over time. After extended use the condition of the finished leathers pigmentation (colour) will be removed by the oil causing the urethane protection to become delaminated.

Leather's greatest enemies are; sun, heat, body oils, perspiration (that contains urea as well as organic salts and acids) and ultra violet radiation (UVR), which dries the hide, fades the colour by bleaching, and can cause the leather to fail by drying out the fibres (dehydration) causing them to shrink as well as the nd the  urethane and / or the hide to crack.  Since body dirt and oil are a big stain factor in leather, be cognizant of bare skin when you are in your vehicle. If you use suntan oil or spray tan lotion, be sure to use a towel when you get back in your vehicle so that the oil does not get onto your leather. You can also use a towel when leaving the gym as body oils/ perspiration contain organic acids that will stain.  

Neat’s-foot, Lanolin (Latin: lāna "wool", and oleum, "oil"), Sperm whale oil, Mink Oil  is a euphemistic name for liquefied pig fat and silicone oil, Tea Tree Oil,  even the so-called Banana Oil, btw it is impossible to get any oil derived from a banana, its real name; Isoamyl acetate is a chemical solvent, often used as a ‘cover-up’ aroma. Silicone and oil- based conditioners are all damaging to urethane coated leather, as they block the movement of moisture back and forth (evaporation and hydration) regardless of what they say on the products label.

Beeswax (or any other organic or inorganic wax) and silicone are hydrophobic, which means they will not allow the replacement of moisture lost through evaporation, nor allow the movement of moisture back and forth.

Silicone again, not ideal for leather in the same way wax or oils aren’t, as they seal the surface and make it slippery, something I personally don’t enjoy in any car. I have seen silicone penetration up close and the only way to remove it from within the hide is to chemically break it down as its function is to impregnate itself into the finish and then form a ‘hard’ shell to protect it.

Silicone oils and waxes also helps attract dust, making  it artificially shiny (losing that OEM matte look)  and can help bring on cracks in polyurethane coated surfaces as it builds up with each coat applied. The net effect of which is dehydration leads to drying and cracking of the surface

Silicon oil formulations are build-up type products which accelerate heat damage, oil formulations are greasy and oily, and have a high electrostatic attraction to dust, grime, which will soil more quickly. We absolutely do not recommend these products for any type of leather application. I have seen silicone penetration up close and the only way to remove it from within the hide is to chemically break it down as its function is to impregnate itself into the finish to harden and "protect" it.

When applied to leather they don’t facilitate the leather or covered lather to receive any hydration, causing it to dry out; it may also have a detrimental effect on the urethane by causing fissure (cracks). Once it permeates the foam and / or hide to remove requires the use of chemicals to break it down as its function is to solidify and form a protective covering, however this will remove the fibre’s flexibility, resulting in a ‘hard’ seating surface

Neat’s-foot oil rots leather is just a myth, any oil will trap moisture in the stitching, which will cause them to fray and rot. If oil is allowed to permeate any micro fissures in the leather or via the stitching it will compromise the resin binder system and delaminate from the hide releasing its adhesive bond, and it will be able to move in a different direction from the hide, which will result in surface fissures and cracking, further compounding the problem eventually leading to the subsequent replacement of the protective covering

If oil is allowed to permeate any micro fissures in the leather or via the stitching it will travel laterally compromising the resin binder system which will delaminate from the hide releasing its adhesive bond. It will then be able to move in a different direction from the hide, which will result in surface fissures and cracking, further compounding the problem eventually leading to the subsequent replacement of the protective covering

Leather is very dynamic with respect to its moisture content; the leather hides needs to be kept supple. The purpose of rehydration is to restore moisture lost through evaporation, so whatever the surface finish, it has to allow the movement of moisture back and forth (evaporation and hydration). The liquoring (fats and oils) that are put into the leather during the tanning process do not dry out of the leather in normal circumstances so therefore do not need replacing.

Modern automotive leather upholstery use a completely different tanning  processes and finishing system, utilizing advanced polymers and chemicals and as a consequence  they do not need to be treated with aftercare products containing oils.

Leather Conditioning 
Modern automotive finished leather upholstery used by 95% of OEM is a multi strata covering over the leather hide; pigmentation (colour) and an abrasion resistant urethane. Finished leather s only requirement is to be kept clean and protected, urethane doesn’t require conditioning

 The following are factual details that leather care manufacturers would rather you didn’t know.
When leather tanners talk about conditioning leather they are referring to re-hydration; not the replenishment or replacement of the fat liquoring oils and waxes. The only 'conditioning' required for finished leather upholstery is hydration; oil-based products cannot permeate the finish leather (urethane pigmentation and / or covering) that is used in 95% plus of modern automobiles.

Modern leather needs to be kept hydrated with moisture to ensure the leather remains flexible and maintains its soft tactile feel. This is done by regularly wiping the surface with a damp 100% cotton micro fibre towel and by using aqueous (water- based) leather care products. There is no reason to use oil-based leather care products to condition or feed leather hides

Aqueous (water- based) products are able to permeate deep into the hide, unlike oil, due to its larger particles, whereas water particles are smaller than both oil and the molecules of urethane, which enables aqueous (water- based) products to permeate and provide hydration, which is essential for suppleness recovery.

Particulate size - you can tell how small the emulsion droplets are and in some cases how concentrated an emulsion is by its colour. Opaque white emulsions typically have a large particle size, while faintly opaque or pearlescent emulsions typically have a small particle size approaching 1µ or less.

           Water - unlike other organic or hydrocarbon-based solvents, is non-flammable, odourless, non-toxic and non-sensitizing to the skin and it doesn’t impart a greasy or tacky feel to the surface of the leather
I have discussed this issue with many people in both the leather tanning and leather care products industry and some specialised industrial chemists who have worked in the leather manufacture and care industry for 35 plus years.

As specialists in leather care they had a much better understanding of what the ideal product is for maintaining finished leather surface used in automotive leather upholstery and i asked the following questions.  

We discussed the product s that are currently being used and the consensus was that many of the products simply were not suitable for the current finishes used for automotive leather

I looked at a detailing care product vendor site and found nine pages of leather care products, mostly expensive oil-based leather ‘conditioners’  this could be the reason they ignore an appropriate care product for the upholstery  material actually used for automotive (finished leather) upholstery. They are in business to make money

1. How much oil-based conditioner or ‘fat liquoring’ will permeate the urethane top coat on a sealed pigmented leather hide? Chrome tanned leather hide is sealed at the tannery and then pigmented; what could a conditioner do for the hide?

2. If oil is allowed to permeate any micro fissures in the leather or via the stitching it will travel laterally compromising the resin binder system which will delaminate from the hide releasing its adhesive bond. It will then be able to move in a different direction from the hide, which will result in surface fissures and cracking, further compounding the problem eventually leading to the subsequent replacement of the protective covering

3. The complex tanning process of chromed tanned hides results in the fat liquoring and oils necessary to keep the hide soft and pliable being locked in, this is further sealed by a durable polyethylene covering to protect the hide from abrasion from clothing as well as the dust / dirt introduced by the vehicle’s AC system.

4. The complaint that most leather conditioners are "greasy" is typically attributable to the use of Lanolin. On most leather conditioners the containers label warns against its use on steering wheels as it will make them slippery and unsafe. The oils cannot permeate the leather and therefore remain on the surface; the same thing will apply to seating surfaces; the problem will be exasperated as the oil will attract dirt/grime to the surface

Many so called leather conditioners utilize chemical solvents in order to facilitate penetration of the oils into the urethane covering or the pigmented leather. Most covered leather finishes are water -based and so any solvent or alcohol can begin to cut through them, even if you go over it and you see no colour come off, you have probably compromised the clear protective top coat and possibly the leather’s pigmentation (colour).

Solvents will soften the protective covering, which can get tacky very quickly, attracting abrasive dust/dirt and will eventually wear through as it does not have the durability found in the topcoat. What happens when the solvents vaporise - polish and many surface protection products are formulated with oils to enhance the surface or to nourish leather surfaces, neither of which is necessary?

Not all conditioners are alike; some are aqueous (water- based) as opposed to oil-based. Some contains about 90% water, when applied to the leather surface, it appears to “soak in” (hydration) leaving only a very thin film of oil to benefit the surface lubrication (driver or passenger entry / exit).

A urethanes fibre structure will stretch in all directions with no particular grain or stress pattern. The urethane surface coating will  not withstand multi directional stress,  however, and when it’s flexed or stretched continuously in the same place the surface coating develops minute cracks. If oil is allowed to permeate any micro fissures in the leather or via the stitching it will compromise the resin binder system and delaminate from the hide releasing its adhesive bond, and it will be able to move in a different direction from the hide, which will result in surface fissures and cracking, further compounding the problem eventually leading to the subsequent replacement of the protective covering

In summary, an aqueous micro emulsion is readily absorbed into the fibres and provides lasting and effective lubrication without migration, while re-hydration leaves leather feeling silky

Three Step Leather Care
Simple cleaning, hydration and protection are the three steps that will prolong the life of finished leather;

Urethane doesn’t require conditioning or rejuvenation; always keep in mind that you’re dealing with the finished coating on the leather and not with the leather hide itself

Clean - there are two cleaning-related factors that can cause your leather to wear prematurely.  The first is dirt, and the second is oil, combined they become very abrasive, as dirt / grit and subsequent friction cause the finish to wear

Hydrated – when leather tanner’s talk about conditioning leather they are referring to its moisture content, re-hydration is used to restore or maintain fluid balance (transpiration and evaporation of moisture); not the replenishment or replacement of the fat liquoring, oils and / or waxes.

Protected - is essential as it will protect the surface finish, without hindering transpiration, while acting  as a sacrificial layer; this way you are not actually cleaning the Leather's original surface, but cleaning from the surface of the protection. It also makes dirt easier to clean off

Ultra violet (UV) protection - 303® Aerospace Protectant will provide invaluable UV-B against photosynthesis (fading) protection; especially in a roadster or convertible vehicle

Associated Articles