Test Your Knowledge Series – Calculating and Measuring Wet Film Thickness

Introduction/Background

Wet film thickness (WFT) is the measured thickness of any applied wet paint that is liquid-based.  The WFT is measured to assure that the correct amount of coating is being applied to the surface.  A wet film thickness gage should be used by the applicator to determine the thickness of the applied film before significant solvent evaporation occurs. Even slight delays in taking wet film thickness measurements can result in false low readings, since the solvents may have evaporated from the film before the measurements are acquired, which is why a WFT gage is largely regarded as an applicator’s tool rather than an inspector’s gage. Measuring the WFT of a coating enables the applicator to adjust the spray gun speed, number of spray-passes to use, and to make spray gun or equipment adjustments to apply the correct amount of coating to achieve the specified dry film thickness.

The coating manufacturer may indicate the range of wet film thickness to be applied to achieve the desired dry film on the product data sheet (PDS) but this is not always the case. Many manufacturers only list the recommended DFT since the amount of thinner that will be added by the contractor is unknown and the amount used effects the target WFT. Project specifications typically list the desired end-result (the DFT) and not the means/methods of achieving it (the WFT). The wet film thickness target (or range) can be calculated using standard formulas: one for un-thinned coatings and one for coatings that are thinned at the time of use.

Using a WFT gage is quite simple. Hold the gage perpendicular to the surface and insert the end into the wet coating. The two end teeth will penetrate down to the underlying surface and will be wetted with coating. There are numbered steps between the end teeth that become progressively deeper. Withdraw the gage and read the highest wetted, numbered step to determine the WFT.  

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