ISO 2808-7B, BS 3900-C5 method 7B, ASTM D4414-A specify that the wet film comb should be perpendicular to the substrate and the thickness of the coating should be stated as lying between the shortest wet tooth and the longest dry tooth.

Wet film combs have teeth of various lengths on their sides.

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 value

Example In the diagram, 90 is the biggest value wet tooth and 100 is the smallest value dry tooth. Therefore, the wet film thickness is between 90 and 100 mil/thou. If the volume to solids ratio of the coating is known (if not contact the coating manufacturer) then the wet film thickness can be used to predict the dry film measurement. To use combs on pipes, measure along the longitudinal axis (lengthways) of the pipe. On rough surfaces, measurements will be made from the surface peaks and represent the minimum wet film thickness.