Wet film thickness measurement and coating quality control

Wet coating thickness measurement is essential during coating construction to ensure that adequate dry coating layers are maintained even after solvent evaporation and curing. According to the guess or approximate amount of paint often lead to coating thickness is insufficient or too thick. Wet film thickness gauge can be used for monitoring the paint coating. However, wet film thickness data would be meaningless if factors such as the specified dry paint thickness and paint volume percentage were not factored into the calculation.

Wet film thickness measurement and coating quality control

The following discussion will explore this computational procedure.

1. The coating composition and wet film thickness calculation

Coatings usually consist of three basic components: resins or adhesives, pigments, and solvents. Resins (binders) and pigments make up the solid part of the paint, while solvents make up the volatile part of the paint. When calculating wet film thickness, the solid volume (the volumetric content of pigment/resin) is an important data, which can usually be obtained from the manufacturer’s product data sheet. Wet film thickness of the basic calculation formula as shown in the equation 9-1.

Equation 9-1 is accurate as long as the solid volume is accurate. However, if the coating by adding diluent, percentage changes. When the diluent was added, the total volume of the material increased, but the solid content did not increase accordingly. Thus, the diluted material will result in a lower solid volume percentage. Therefore, when comparing diluted to undiluted materials, a heavier wet coating of the diluted material is required in order to achieve a comparable dry coating thickness.

2. The wet film thickness measurement considerations

The use of wet film thickness meters relies on solid volume, which is considered as the “in-tank” percentage. Therefore, the wet film thickness measurement should be carried out as soon as possible when the coating is applied to the surface. In the process of spraying, paint spray gun from left to the coating to the surface, some solvent evaporation may have slightly changed the solid volume percent. For most applications, this change is not significant; However, for coating materials with fast solvent evaporation rates, such as vinyl and chlorinated rubber coatings, the measurement of wet film thickness may not be meaningful. The longer the time elapsed before the measurement, the less accurate the measurement. Coating for high pigment content (such as zinc rich coating), wet film thickness measurement may not be reliable, because although coating contains a certain percentage of the solvent, but as a result of solvent loss caused by the contraction may be very small. Obviously, on 100% solid content of coatings, the wet film thickness will be equal to the dry film thickness.

Wet film thickness measurement and coating quality control

3. Wet film thickness measurement tool

Wet film thickness gauge usually has a standard indentation structure. The notch thickness gauge consists of two end points located in the same plane with gradually deepening notch steps in the middle. Each step is represented by a number indicating the distance between the step and the plane formed by the two endpoints, usually in mil or micron. Press the instrument firmly perpendicular to the substrate into the wet paint before pulling it out. Two endpoints will be wetting coating material, and some of the steps. The wet film thickness is considered as the last wetting step before the next adjacent, higher drying step.


Wet paint coating thickness measurement and protection ability has an important influence on the performance of the coating. By accurately calculating the wet film thickness, it can be ensured that the coating has an appropriate dry coating layer after drying and curing. When the wet film thickness measurement, however, must take account of coating composition and its factors such as volume percent. With proper measurement and calculation, accurate guidance can be provided for coating construction to ensure the quality and performance of the coating.

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