Wet Film Thickness or WFT is the measured thickness of any applied liquid-based wet paint. When coating, the applicator should use a wet Film Thickness Gauge to ensure that measurements represent the calculated wet film before significant solvent evaporation occurs. When taking wet film thickness measurements, even a slight delay can result in falsely low readings because the solvent may have evaporated from the film before the measurement is taken, which is why WFT gauges are largely viewed as applied Tools, not inspectors.
Why is WFT important? By measuring the WFT of the paint, the applicator can adjust the gun speed, number of coats, and make gun adjustments (if possible), or select a different spray head to apply the correct amount of paint to achieve the specified dry film thickness.
What is the relationship between WFT and production? time is money. It may be necessary to apply build-up coats, or even worse (by sanding) to reduce thickness, which hinders production and reduces profitability. While insufficient thickness can usually be corrected by adding more coats, excessive thickness can lead to solvent entrapment, sagging and sagging which, if not corrected, can lead to adhesion problems. Each of these events can negatively impact project schedules. Perhaps even more important to remember: The performance of most coatings is based on achieving a specified dry film thickness, and applying the correct wet film thickness can help meet this requirement. Measuring wet film thickness during application can immediately determine that applicators need to make in-process adjustments.
How is WFT calculated? Paint manufacturers may specify on the Product Data Sheet (PDS) the range of wet film thicknesses to obtain the desired dry film. However, many manufacturers only list a suggested DFT because the amount of diluent that the contractor will add is unknown and that amount affects the target WFT. Specifications usually list the desired end result (DFT), not the method/approach (WFT) to achieve this. A wet film thickness target (or range) can be calculated. Formulas to calculate WFT with and without diluent addition are shown. The dry film thickness range (per coat) is extracted from the specification or the paint manufacturer's PDS (whichever is the control document), and the solids content is listed on the PDS.
What are the volume solids in paint? An expression of the film-forming component of the S content of a coating in Volume Solids, or the solvent left after evaporation of the material from the applied coating. On a very basic level, paint consists of solvents, resins, pigments and additives. Volume solids is the percentage of formulation that is non-volatile and will remain on the surface after the coating has dried and cured.
Without thinner:
湿膜厚度（WFT）=干膜厚度（DFT）÷体积百分比固体
例：
规定的干膜厚度= 3-5密耳
体积固体含量= 65％（0.65）
WFT = 3÷0.65 = 4.6密耳; 5÷0.65 = 7.7密耳
According to该示例，假设施加器施加的WFT为5到8密耳，则应达到3-5密耳的指定DFT。
用于计算在车间或现场添加了稀释剂的目标WFT的方程式要求，需要According to添加的稀释剂的体积（占涂层总体积的百分比）调整涂层（制造时）的体积固体含量。
随着稀释剂：
WFT = DFT÷（体积固体含量÷100％+添加的稀释剂的百分比）
例：
规定的干膜厚度= 3-5密耳
体积固体= 65％
体积稀释剂= 20％（例如，将2加仑的稀释剂加到10加仑的混合涂料中）
步骤1：（0.65÷1.20）= 0.54，或基于20％稀释剂添加量的54％调整体积固体
步骤2：3÷（0.54）= 5.6密耳5÷（0.54）= 9.3密耳
According to该示例，假设施加器施加的WFT在6到10密耳之间，则即使达到20％的稀释度，也应达到3-5密耳的指定DFT，这是湿膜而不是干膜的一部分。
WFT如何测量？湿膜厚度计用于监控所施加的湿涂层的厚度，以达到指定的干膜厚度。它们可测量所有类型的湿有机涂料，例如在平坦或弯曲，光滑表面上的油漆，清漆和清漆。这些量规的测量单位通常为微米（mil）或密耳。湿膜厚度ASTM D4414，通过缺口规测量湿膜厚度的标准实践来测量。图片是通常用于测量WFT的量具。
How are WFT gauges used? Using the WFT gauge is very simple. First, verify that the notches (teeth) are clean and free of any dried paint. Immediately insert one end of the gauge vertically into the wet coating. The teeth at both ends will penetrate down to contact the underlying surface and be wetted by the coating. Take out the gauge and read the highest wetness step. If none of the numbered notches contain wet coating, turn the head of the gage to the lower WFT range and measure again. If all numbered notches contain wet coating, rotate the head of the gage to a larger WFT range and re-measure. A schematic depicting the proper use of the WFT gauge is shown. After each reading, the teeth of the gauge need to be wiped.
What units are used when measuring WFT? Typical units of measurement are mils and microns. A mil is a unit of length equal to one thousandth (10 -3) of an inch (0.0254 mm). A micron is a unit of measurement for a length equal to 0.001 mm (approximately 0.000039 inches). Its symbol is µm. 25.4 microns equals 1 mil.
Conclusion: Calculating and properly measuring wet film thickness can reduce rework, increase productivity, and help ensure that coatings are applied correctly. The importance of proper use of the WFT gauge at the coater to achieve the desired dry film thickness cannot be overemphasized.
