Wet Film Thickness – Measurement Before Cure
Wet-film thickness gauges help determine how much material to apply wet to achieve a specified dry-film thickness provided that the percent of solids by volume is known. They measure all types of wet organic coatings, such as paint, varnish, and lacquer on flat or curved smooth surfaces.
Measuring wet film thickness during application identifies the need for immediate correction and adjustment by the applicator. Correction of the film after it has dried or chemically cured requires costly extra labor time, may lead to contamination of the film, and may introduce problems of adhesion and integrity of the coating system.
The equations for determining the correct wet-film thickness (WFT) and dry-film thickness (DFT), are as follows:
WFT = 1600 ÷ Application Rate of the product used.
DFT = WFT ÷ % of solids by volume
Wet-film is most often measured with a wet film comb or wheel. The wet-film comb is a flat aluminum, plastic, or stainless steel plate with calibrated notches on the edge of each face. The gage is placed squarely and firmly onto the surface to be measured immediately after coating application and then removed. The wet-film thickness lies between the highest coated notch and the next uncoated notch. Notched gage measurements are neither accurate nor sensitive, but they are useful in determining approximate wet-film thickness of coatings on articles where size and shape prohibit the use of more precise methods. (ASTM D1212).
The gage should be used on smooth surfaces, free from irregularities and should be used along the length, not the width, of curved surfaces. Using a wet-film gage on quick-drying coatings will yield inaccurate measurements. ASTM D4414 outlines a standard method for measurement of wet-film thickness by notch gages.
A wet film wheel (eccentric roller) uses three disks. The gage is rolled in the wet film until the center disk touches the wet film. The point where it makes contact provides the wet film thickness.