The hardness of the paint film is important, because it is related to brittleness and water permeability. Films that have been over-cured or under-cured can often be detected by hardness testing. Pencil hardness and indentation hardness are two widely used testing techniques.
Pencil hardness tests measure resistance to indentation by a series of increasingly hard pencils that have been sharpened to a chisel point. The higher the hardness of the pencil lead required to make a gouge into the paint, the harder the film.
Indentation hardness testers measure film penetration, in a specified time, by a metal tip under certain mechanical loading conditions. The time factor is important, since many paints deform gradually under load.
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