The knife or coin test gives an approximate idea of the hardness of a paint film. Controlled scratch testing and rocker pendulum tests are more used for more quantitative measurements. The scratch test is applied to a test panel carrying the fully hardened paint coating, a needle with a rounded point is placed on the paint film and the panel is dragged slowly beneath the needle.

The test is made with a step-wise increase-of weights on the needle and the weight producing the first scratch of the film is taken as a measurement of hardness.

The rocking pendulum type of apparatus consists of a wheel with hardened steel rims which is set rocking on a paint coated panel. and a measurement is made of the time taken for the instrument to come to rest.

The time interval is compared with the time taken for the completion of a similar movement of the instrument on a standard plate glass surface. The paint film has a dampening effect on the swings of the rocker, the degree of dampening lessening with increasing hardness of the film.

Indentation hardness is another means of assessing hardness, this involves I placing a hard diamond shape onto the paint surface, applying a known weight for a standard time and then measuring the depth or length of the indentation left in the paint surface; elastic recovery of the paint film may also be measured with this apparatus.

A range of pencils of varying hardness can be employed as a means of assessing approximately the hardness of a paint film. The grade of pencil which 1 succeeds in marring the paint film which the pencil point is drawn firmly across the film serves as a hardness index for the film. Hardness may range from H12 (hardest pencil lead) through HB to B12 (softest).