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Paint Density (Hardness)

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Posted 20 July 2011 - 04:04 AM

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 and the paint drying process used.

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.

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.

Low paint density (soft paint) could also be caused if the clear coat was applied before the base coat paint had time to fully cure

Surface Hardness

Is defined as 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 white single stage and CeramiClear, when compared to all the materials your paint can possibly come in contact with (always bear that in mind).
On the Mohs scale, graphite (a principal constituent of pencil "lead") has a hardness of 1.5; a fingernail, 2.2–2.5; a copper penny, 3.2–3.5; a pocketknife 5.1; a knife blade, 5.5; window glass plate, 5.5; and a steel file, 6.5.[9] A streak plate (unglazed porcelain) has a hardness of 7.0. Using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale.

An adaptation of that hardness scale (1 - 10)

• Talc = 1H
• Carbon Black [black paint pigmentation] = 2H
• Glass = 6H
• Titanium dioxide [white paint pigmentation] = 7H
• Corundum [~nano coatings] = 9H
• Diamond =10 H

Coating Products

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 pencil hardness alone, as there are many other significant characteristics to consider.

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 (after 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

Pencil Hardness - the test uses special pencils with different degrees of hardness to scratch the coating, which then determines its hardness - Testing Your Coating's Hardness

Relative Hardness

Hard and soft are both relative terms; you can scratch the hard surface of a 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 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)

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Edited by TOGWT, 21 July 2011 - 07:22 AM.

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