Specified Dry Film Thickness = 3 – 5 mils Wet Film Thickness (WFT) = Dry Film Thickness (DFT) ÷ Percent Solids by Volume The volume solids content is the percentage of the formulation that is non-volatile and will remain on the surface after the coating dries and cures. On a very basic level paint contains solvent, resin, pigments, and additives. What are volume solids in paint? The v olume solids content of a coating is an expression of the film-forming ingredients, or the material left behind after the solvents have evaporated from the applied coating. The dry film thickness range (per coat) is extracted from the specification or the coating manufacturer’s PDS (whichever is the governing document) and the volume solids content is listed on the PDS. The equations for calculating the WFT, both with and without thinner addition, are shown. The wet film thickness target (or range) can be calculated. Specifications typically list the desired end-result (the DFT) and not the means/methods of achieving it (the WFT). However, many manufacturers only list the recommended DFT since the amount of thinner that will be added by the contractor is unknown and that amount effects the target WFT. How is the WFT calculated? 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). Measuring wet film thickness during application immediately identifies the need for in-process adjustments by the applicator. Perhaps more importantly, the performance properties of most coatings are based on achieving the specified dry coating thickness, and applying the correct wet film thickness can help to meet this requirement. Each of these events can negatively impact a project schedule. While under-thickness can frequently be corrected by adding more coating, excessive thickness can cause solvent entrapment, runs and sags and, if uncorrected can lead to adhesion problems. ![]() The need to apply a build-up coat or even worse the need to reduce thickness (by sanding) can impede production and reduce profitability. What is the relationship between WFT and production? Time is money. ![]() Why is WFT important? Measuring the WFT of a coating enables the applicator to adjust the spray gun speed, number of spray passes and to make spray gun adjustments (when possible) or select other spray tips to apply the correct amount of coating to achieve the specified dry film thickness. 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 applicators tool rather than an inspector gage. A wet film thickness gage should be used by the applicator as the coating is being applied to ensure that the measurement is representative of the calculated wet film before significant solvent evaporation occurs. ![]() Wet film thickness, or WFT is the measured thickness of any applied wet paint that is liquid-based.
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