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]
Modern clear coat urethane even though they are harder than lacquer paints, they still scratch more easily. And because they tend to be harder it's more difficult and time consumer to remove swirls and scratches. This is the main reason to take vendor claims of ‘provides 9H hardness ‘for what it is – totally irrelevant.
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)
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 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]
It is important to note that manufacturer-approved paint suppliers vary by country as each country (and even province/state) has their own environmental laws they must adhere to.
Paint is graded by its hardness or density by a Durometer; one of several measures of the hardness of a material. Durometer, like many other hardness tests, measures the depth of an indentation in the material created by a given force. Clear coats can be made with harder or softer durometer and that will also determine its scratch resistance.
Pencil hardness test is one of many non-Scientific 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, the VOC and HAP contents, and so on.(See also American Society for Testing and Materials (ASTM) publications)
Pencil Hardness Tester
The pencil hardness test, also referred to as the Wolff-Wilborn test, uses the varying hardness values of graphite pencils to evaluate a coating’s hardness.
The Elcometer 501 Pencil Hardness Tester has been designed to ensure that the cylindrical pencil lead is maintained at a constant angle of 45° and exerts a force of 7.5N (1.68lbF).
The pencil lead, prepared beforehand using the special sharpener and abrasive paper, is inserted into the Elcometer 501 hardness tester and pushed over the smooth, flat coated surface.
The lowest hardness value of the pencil which marks the coating determines the coating’s hardness rating.
After doing some testing on various coatings for their 'pencil hardness'. The hardest is a 9H, followed by 8H, 7H, 6H, 5H, 4H, 3H, 2H, and H. F is the middle of the hardness scale; then comes HB, B, 2B, 3B, 4B, 5B, 6B, 7B, 8B, and 9B, which is the softest.
Does paint density (hardness) affect its scratch resistance? - many coating products claim an obtained surface (Pencil) hardness of 9H 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 coating network as well as the surface’s friction resistance.