These charts have a unique surface which is readily wetted by waterborne or solvent borne paints, but from which the dried film can easily be stripped with adhesive tape although not so easily as to produce undamaged free films. The stripping feature permits the dry film weight on a measured area to be determined precisely by weighing on an analytical balance before and after film removal. The spreading rate (H) and wet film thickness (T) can then be calculated from the following simple relationships:
H(m²/L) = [A (cm²).N.D(kg/L)] / 10M(g)
H(ft²/gal) = 40.746 H(m²/L)
H (ft²/gal) x T (mils) = 1604.2
H = spreading rate (m²/L), (ft²/gal).
T = wet film thickness (µm), (mils)
A = test area (cm²)
D = paint density (kg/L)
M = dry film weight (g)
N = non-volatile fraction by weight of the applied paint
This procedure represents a breakthrough in reduced time for precise measurement of spreading rate (or wet film thickness) in the determination of hiding power.