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 But rightly due to environmental concerns, those high percentage petroleum based paints have been generally superseded, resulting in the softer water-based paint finishes of today and the unavoidable orange-peel seen on many new and re-painted vehicles.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).
Mohs scale of mineral hardness
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. 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 9H
• Diamond =10 H
Pencil Hardness - the test uses special pencils with different degrees of hardness to scratch the coating, which then determines its hardness - http://www.woodweb.com/knowledge_base/Testing_Your_Coatings_Hardness.html
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 (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
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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