Wet film combs are available in a variety of materials including stainless steel, aluminium and plastic and with various measurement scales ranging from 20µm to 3000µm.
Using a wet film comb: place a comb perpendicular to and touching the substrate. Hold the comb in position and wait a few seconds until the teeth are wet. Remove the comb from the film. The wet film thickness lies between the biggest value ‘coated’ or ‘wet’ tooth and the smallest value ‘uncoated’ or ‘dry’ tooth.
For more information, see the Elcometer Guide to Wet Film Thickness.