One of the most often asked questions; “What polish / pad combination should I use to remove scratches?”
There are far too many variables to state this accurately.
Diagnosis is the key, not guess work; (see Test Panel Area) before deciding on what products to use ascertain the paint surface conditions and your goals for the paint finish. And then proceed with the least abrasive product / pad combination
Differing density (hardness) of paints react differently to correction, so before beginning to polish measure the paint thickness (PTG) panel by panel to ensure that you do not select an aggressive polish that will remove too much clear coat
Compiling a chart that lists the consistency of automotive paint can only at best be a rough guide, as this would necessitate listing by; manufacturer, OEM assembly plant, model year, colour, new or aged, paint specification, and etc. To ascertain paint hardness requires extensive experience working on a wide range of vehicles to have a point of reference.
Identical vehicles of the same make and model/year can react very differently to compounds/polishes/etc. There are too many variables (paint spec, curing method, paint age, etc.)
Hardness: [In materials science, hardness is the characteristic of a solid material expressing its resistance to permanent deformation. There are three principal operational definitions of hardness: Scratch hardness, Indentation hardness, Rebound or dynamic hardness] 
Hardness is dependent on ductility, elasticity, plasticity, strain, strength, toughness and viscosity; suffice to say it is extremely important that paint hardness is properly taken into account when polishing, because if it is not then inappropriate polishing products and pads may be selected that either remove too much paint or turn out to be completely ineffective
Hard and soft are relative terms; you can scratch the ‘hard’ surface of your car’s paint finish with a ‘soft’ towel with the application of sufficient (localized) 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]
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.
Scratch resistance can be related to higher cross-link density and elasticity of the polymer network The theory is that a dense cross-linked (hard) paint provides better protection from scratches and stone chips, whereas a less dense (soft) paint absorbs the impact.
In solid mechanics, solids generally have three responses to force, depending on the amount of force and the type of material:
[: any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film]
Paint protection coatings have many similarities to automotive paint and share the same solids mechanics attributes
Paint is a liquid, usually comprising 30 to 40% solids and 60 to 70% liquid. As it dries it becomes a solid and a gas (the solvent) that evaporates, leaving behind the solid. The three primary type of ingredients found in most paint type coatings are (1) Binder (film former) (2) Carrier (solvent) (3) Solutes (pigment). The binder is the film-forming component of paint, it imparts properties such as gloss, durability, flexibility, and toughness. Many binders are too thick to be applied and must be thinned. The thinner is also called the carrier, because it makes it possible to transfer the binder to the surface. The pigment provides the paint with colour.
All paints consist of a solids portion, essentially comprised of the pigments and binder, and a liquid portion. After the liquid portion evaporates and a coating dries, it is the solids that remain on the surface. Some paints form by solvent evaporation only, but most rely on cross-linking processes
Surfactants are added to stabilize the paint so that it will not separate, settle or become too thick to use. They also keep pigments dispersed for maximum gloss and hiding; and they help “wet” the surface being painted so the paint won’t “crawl” when it is applied. Surfactants also provide compatibility between tinting colorants and bases so that the correct colour will be obtained.
Solid mechanics [: solids generally have three responses to force, depending on the amount of force and the type of material]
· Elasticity - the ability to temporarily change shape, but return to the original shape when the pressure is removed.
· Hardness (in the elastic range) -a small temporary change in shape for a given force, is known as stiffness in the case of a given object, or a high elastic modulus in the case of a material.
· Plasticity - the ability to permanently change shape in response to the force, but remain in one piece.
· Yield strength - is the point at which elastic deformation gives way to plastic deformation.
· Deformation (in the plastic range) - is non-linear, and is described by a stress-strain curve. This response produces the observed properties of scratch and indentation hardness, as described and measured in materials science.Some materials exhibit both elasticity and viscosity when undergoing plastic deformation; this is called viscoelasticity..
Low cross-link densities raise the viscosities of the polymer (soft). Intermediate cross-link densities transform gummy polymers into materials that have elastomeric properties and potentially high strengths. Very high cross-link densities (hard) can cause materials to become very rigid with a high gloss
Paint density is determined primarily by OEM paint specification requirements, the variables are; the type of resin used, the type of hardeners and other additives, types of solvents used and the paint drying process used.
Somewhat surprisingly, paint hardness actually varies significantly between different makes of car, and also in some cases between different ages of the same make and model of car. The reasons why this is so are quite complicated, but suffice to say it is extremely important that paint hardness is properly taken into account when polishing, because if it is not then inappropriate polishing products and pads may be selected that either remove too much paint or turn out to be completely ineffective .
A unique aspect of polyurethane chemistry is that the hydrogen bonding acts as an additional crosslink, but also allows thermoplastic flow, which helps the paint surface to retain its elasticity and its tensile strength to relieve mechanical stress.
The basic structure of a polyurethane clear coat features a soft segment (polyol or tetra methylene ether) which gives it flexibility and elasticity. There is also a hard segment (polymerization) that has high urethane density, which gives the coating hardness and tensile strength
In polymer chemistry, when a synthetic polymer is said to be "cross-linked", it usually means that the entire bulk of the polymer has been exposed to the cross-linking method. The resulting modification of mechanical properties depends strongly on the cross-link density
Each vehicle assembly plant uses different clear coat paint from one of three major paint suppliers; PPG, DuPont, and BASF products and each of these companies have a range of several differing paints
Most vehicle manufacturers will issue a painting specification that denotes amongst other things; a paint to hardener ratio, dependent upon quality control this spec may or may not be strictly adhered to
Some vehicle manufacturers have more than one plant assembling the same model of a vehicle; each plant will often use a different supplier for the clear coat paints.
Each assembly plant may elect to use one of several OEM paints from PPG, DuPont or BASF clear coat product lines.
This is also true of imports assembled in the US; usually one paint supplier is approved for all plants; however each plant may modify the application/bake process in order to meet production demand. This may also affect which has the harder or softer clear.
There are other factors that will have an effect; the composition of the clear coat used (single, duel component, or powder) this generally reflects the trade-off the OEM is prepared to accept between scratch resistance and gloss level considered acceptable, oven drying time and its temperature, quality of the quality of the isocyanates used, relevant age (i.e. how long ago was the paint applied) spot panel repairs (refinish) that are carried out either at the assembly plant or the rail head or port of entry. Paint density (soft paint) could also be caused if it was polished before the paint had time to fully cure
To date only water-based paints are used for the base coat, the clear coat is still a VOC compliant, low ratio solvent- based paint, which will be denser, dependent upon the quality of the isocyanates used. There are also a couple of other factors;
OEM use a different resin (Melamine) which must be heat cured (300.0F) which produces a denser (hard) paint that tends to be more difficult to polish, whereas body shops use an acrylic resin based clear coat.
Paint can be very complex and yet really interesting; in polymer chemistry, when a synthetic polymer is said to be cross-linked, it usually means that the entire bulk of the polymer has been exposed to the cross-linking method. The resulting modification of mechanical properties depends strongly on the cross-link density.
This is how tight or dense the paint matrix becomes after is it fully cured, think of a paint matrix like that of chicken wire mesh, a small mesh (dense) larger mesh...,
I think you get the analogy. Hard paint has a dense cross-link density, whereas low density (soft) paint has a less cross-link density.
Specific gravity [: the ratio of the density (mass of a unit volume) of a substance to the density (mass of the same unit volume) of a reference substance] using water as a reference it’s SG is 1.00 at 20.OC, a substance with a relative density (or specific gravity) less than 1.0 will float. Relative density (RD) or specific gravity (SG) is a dimensionless quantity, as it is the ratio of either densities or weights
Paint density - fumed silica is often used as a hardener; it has a low specific gravity and rises to the upper layers of the paint. This is part of the reason for clear coats being ‘soft’ below 7.5 µ (0.3 Mil) fumed silica paint hardeners are evenly distributed throughout the clear when applied, however, as part of the curing process; the majority migrate to the top of the clear coat. This is why many paint renovation detailers observe a less dense (softer) clear after the removal of 12.5 µ (micron) or 0.5 Mil.
The reasons why this is so are quite complicated, but suffice to say it is extremely important that paint hardness is properly taken into account when polishing, because if it is not then inappropriate polishing products and pads may be selected that either remove too much paint or turn out to be completely ineffective .
Scratch resistance - can be related to higher cross-link density and elasticity of the polymer network. Low cross-link densities raise the viscosities of the polymer (soft). Intermediate cross-link densities transform gummy polymers into materials that have elastomeric properties and potentially high strengths. Very high cross-link densities (Hard) can cause materials to become very rigid with a high gloss
Dense paint is very resistant to correction; some paints will correct with a single pass and look incredible, ready for wax even. Others will correct just as easily butleave a hazy appearance that needs to be cleaned up with a less aggressive polish and a soft pad.
The best way to detail paint, wither it is hard or soft (dense or less dense); is by using the least aggressive pad / polish combination followed, if necessary by an incrementally more aggressive approach until you find what works.
Dense (Hard) paint - paint requires a more aggressive abrasive polish and takes more time to correct. But a dense (hard) paint makes it easier to get that perfect gloss. PPG's Optech™ or CeramiClear™ respond better to polishes / compounds formulated for this type of paint density.
Less dense (Soft) paint- responds to abrasives more readily and makes polishing easier, but it’s easier to induce surface marring.
• Water-based paint is generally more porous and less dense (softer)
• Solvent-based clear coat paint is usually more dense (harder) than a water-based paint and not as porous
As always generalizing carries a degree of risk as a panel(s) may have been re-painted. Before commencing polishing do a test panel on the car, once you have achieved the desired results with your selected polish / pad combination proceed
PPG's Optech™ or CeramiClear™ respond better to polishes / compounds formulated for this type of paint density.
Paint is graded by its density by a Durometer; one of several measures of the hardness of a material. Like many other hardness tests, it measures the depth of an indentation in the material created by a given force. Clear coats can be made with harder or softer densities and that will also determine its scratch resistance.
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 Mohs scale of hardness, especially single coat black / red paint the exception being white single stage and PPG's Optech™ or CeramiClear™, when compared to all the materials your paint can possibly come in contact with (always bear that in mind).
Mohs Comparative Hardness Scale
An adaptation of that hardness scale (1 - 10)
1. Talc = 1
2. Carbon Black [black paint pigmentation] = 2
3. Glass = 6
4. Titanium dioxide [white paint pigmentation] = 7
5. Diamond =10
It is extremely important that paint hardness is properly taken into account when polishing, otherwise inappropriate polishing products and pads may be selected that either remove too much paint or turn out to be completely ineffective
Diagnosis is the key, not guess work; before deciding on what products to use ascertain the paint surface conditions and your goals for the paint finish. And then proceed with the least abrasive product / pad combination
The Pencil Hardness Test provides a simple method to test the scratch hardness of coatings. In this test, pencils in a range of 6B to 8H hardness-grade are used. Pencils graded using this system is used to measure the hardness and resistance of varnishes and paints. The resistance of a coating (also known as its pencil hardness) is determined as the grade of the hardest pencil that does not mark the coating when pressed firmly against it at a 45 degree angle.
Select a pencil and make a line about one inch long on the paint sample. If the pencil leaves a scratch, then take the next softer pencil and do the same thing. The number of the first pencil that you use after you have made a scratch in the coating that doesn't leave a mark is considered the "pencil hardness" of the coating. (There are some coatings that are so hard that even the 9H pencils will not scratch them. All of those coatings get a 9H pencil hardness rating to designate their hardness.)
Pencil Hardness for Common Coatings
· Catalysed polyurethane: 9H
· Catalysed modified acrylic polyurethane: 4H
· Catalysed acrylic polyurethane: 2H
· Water-based polyurethane: 3H
· Water-based urethane/isocyanate catalyst: 2H
· Low-VOC lacquer: 3H
· Low-VOC catalysed lacquer: 2H (24 hours)
· Urethane/nitrocellulose lacquer: F (24 hours)
· Clear shellac aerosol: 3B
· Polyurethane/nitrocellulose aerosol: HB
Hard / Soft Paint Correction
Experienced detailers can evaluate paint condition quickly and accurately but they still perform a test spot to make sure they choose the right methodology. Experience with different OEM paint density and colour will teach you that no two vehicle paints, even the same marque and paint colour are the same. Paint correction is a complex process with many variables; polisher, type and abrasive level of pads and polish or compound, speed and pressure used, and etc.
Dense (Hard) paint - is very resistant to correction; if you see very little change on your ‘test’ providing you are using an appropriate polish / pad combination and proper technique and you’re seeing very little correction you've got fairly hard paint.
When dealing with less dense (soft) paint, your ‘test panel’ looks far from perfect most neophyte’s are under the impression that they need to get more aggressive. Nothing could be further from the truth, even though the finish may not look all that great, it is an improvement from where they started. Usually a less dense or intermediate density (soft or medium soft) paint will result in really bad hazing.
Really dense (hard) paint (DuPont's SupraShield™, PPG's Optech™ and CeramiClear™) - will rarely haze; it’s usually very unresponsive but will show signs of improvement after a few passes
Some paints will finish with just a single pass; others will finish with a ‘haze’ that requires that you 'finish' with a less abrasive pad / polish combination. There are some paints that have a very low density (very soft) paint that allow swirls and micro marring, etc. to be removed easily but it just appears dull and flat no matter what you do. A very gentle approach is called for in those cases, requiring a uniformly applied pressure across the pad / backing plate
Does paint density (hardness) affect its scratch resistance?
Many coating products claim an obtained surface hardness of 9H or 3 Gpa+, but this has little if anything to do with scratch resistance. Scratch resistance can be related to higher cross-link density and elasticity of the polymer network.
So how can a dense (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. (Newton's third law of motion)
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).
This can best be summed up as: your car’s paint finish, though it may feel hard, is actually rather soft. Nearly everything else that comes in contact with it, though it may feel soft, is harder by comparison and will scratch it with enough pressure – sometimes just the slightest of pressure.
Test Panel Area
Diagnosis is the key, not guess work; before deciding on what products to use ascertain the paint surface conditions and your goals for the paint finish. And then proceed with the least abrasive product / pad combination
Modern automotive paint systems of whatever density (hardness) pose unique challenges to the detailer, to determine the correct polish/and pad combination, machine methodology. This requires a ‘test panel’ to see how the selected polish set-up responses to the paint, and wither it will remove the surface scratches
Select a ‘typical’ 18 x 18 – inch area -one that represents the type of defects that you want to remove and starting with the least abrasive polish and / pad combination and then proceed until the defects are removed. This is the polish / pad combination you’ll need to use. Selecting the correct pad / polish combination for the vehicles paint / defects can take just as long as the paint correction process
Science of Scratches
Dictum [: a universally accepted principle or rule]
In a clash between two materials the harder material will come out unscathed – unscratched – and the softer material will wear the battle scars.
Hard and soft are relative terms; you can scratch the “hard” surface of your car’s paint finish with a “soft” towel if I apply enough (localized) pressure.
This one is really short and simple and rather obvious. Heat makes solids softer; cold makes them harder. Keep that in mind when detailing your car in the sun or on a really, really hot day.
Glidden, Nason, Sherwin Williams, Sikkens, PPG Protective and Marine Coatings, DuPont™ Performance Coatings, Spies Hecker® (part of DuPont, used mostly by European auto manufacturers) and BASF coatings, each of these companies have a range of several differing paints.
Paint is a simple mixture of ingredients including pigments, a binder and diluents, or thinner (in a water-based paint it is water; in a solvent-based paint, it is petroleum solvent). The clear coat also has ultra violet protection added to protect the paint matrix.
Automotive paints utilizing modern paint technology using water-based high solid/low solvent urethane that for environmental reasons produce a much softer and more porous finish than acrylic or oil based high solvent content paint systems.
Using permanent pigmentations and colorants specifically designed for automotive applications (containing automotive-grade binders). Albeit a non-toxic (less than 0.01-percent VOC) water-based paint, there are no compatibility issues with major-brand urethane and polyurethane clear coats.
Nano technology Coatings
DuPont's SupraShield™, PPG's Optech™ and CeramiClear™ Clear Coat are all binary clear coat compositions using ceramic particles designed to offer long-term scratch resistance, gloss and durability
Working with PPG, BASF and Mercedes Benz, in 2003 Menzerna developed special polishes for use on PPG CeramiClear™ Clear Coat for removing scratches, swirls and paint defects Beyond superior abrasives, Menzerna has pioneered the development of polishes designed specifically for the hard clear coats, like those used by General Motors on the Corvette and PPG CeramiClear™ Clear Coat being used by Ferrari and Mercedes- Benz.
Super Intensive and Nanotechnology Polish are currently used by Mercedes - Benz in Germany on their production line to remove swirls and over-spray incurred during the painting process. Some OEM paint specification require a dense (hard) clear coat paint- Audi, BMW, Nissan and VW and others.
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.
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