The Physics of Polishing

Physics (from Ancient Greek: φύσις physis "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.]

 Backing Plate / Pad Motion

The benefits of a large orbit or elliptical offset 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; therefore levelling is more consistent. This type of pad movement helps to remove residue (oxidized paint, spent abrasives, etc) more readily than a small elliptical offset

A majority of random orbital machines use an elliptical offset between 1/8 -5/16 - inch (approximately 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

Surface (Contact) Area

The distance around the circle is a circumference. 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 = π 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 = (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 – 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 = [π (radius²)] / [π (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” 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

Kinetic Friction

 [ : when contacting surfaces move relative to each other, the friction between the two surfaces converts kinetic energy into thermal energy, or heat]

Friction is the force resisting the relative lateral (tangential) motion of solid surfaces, fluid layers, or material elements in contact. It is usually subdivided into several varieties:

Dry friction is also subdivided into static friction between non-moving surfaces, and kinetic friction (sometimes called sliding friction or dynamic friction) between moving surfaces.

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.

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] [1]

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, besides the most commonly used abrasives include varieties of aluminium oxide, silicon carbide, diatomaceous earth, clay, and silica, to produce enough heat to cause a reduction in size would harm the paint. Kinetic friction is required to ‘level’ paint, which is simply the removal of paint to the lowest point of the paint defect.

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.

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.

Polishes and compounds do not need heat per se for the abrasives to polish a surface, wither 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.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) [/I]

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 ideal Gas Law 

Boyle's law and Charles's law, known as the ideal gas law is often introduced in its common form (pV = nRT), which states that  heat makes the gasses expand, and 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 (elastomer) 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 infra-red ‘gun’ thermometer, paint surface ‘spot’ temperature should be limited to 110.^oF <

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 = [π (radius2)] / [π (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.

Mohs relative hardness

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.

Relevant Articles

1.      “Paint (Solvent / Alcohol) Swelling “-http://www.autopia.org/forum/autopia-detailing-wiki/136812-paint-solvent-alcohol-swelling.html

2.      “Wipe down (Scratch lie detector test) Process” - http://www.autopia.org/forum/autopia-detailing-wiki/137115-wipedown-scratch-lie-detector-test-process.html