Wet film thickness measurement and coating mass control

During Coating construction, the thickness measurement of wet Coating is critical to ensure that sufficient dry Coating layers remain after solvent evaporation and curing. Coating amounts based on guesses or approximations often result in insufficient or excessive Coating thickness. The Wet Film Thickness Gauge can be used to monitor the amount of Coating applied. However, wet film thickness data is meaningless without factoring in factors such as the specified dry Coating thickness and Coating volume percentage. 

The discussion below explores this calculation process.

1. Coating composition and WFT calculation

Coatings usually consist of three basic components: Resin or gluing agent, pigment and solvent. Resin (gluing agent) and pigment constitute the solid part of the Coating, while solvent constitutes the volatile part of the Coating. Solids by volume is an important data when calculating wet film thickness, which can usually be obtained from the manufacturer's product data table. The basic formula for calculating wet film thickness is shown in Equation 9-1.

Equation 9-1 is accurate when solids by volume are accurate. However, if thinner is added to the Coating, the percentage changes. When thinner is added, the total volume of the material increases without a corresponding increase in percent solids. Therefore, the diluted material will result in a lower percentage of solids by volume. Therefore, when comparing diluted to undiluted materials, in order to achieve a comparable dry Coating thickness, a heavier wet Coating coating of the diluted material is required.

2. Precautions for wet film thickness measurement

The use of Wet Film Thickness Gauge relies on solids by volume, which is considered an "in-tank" percentage. Therefore, wet film thickness measurement should be performed as soon as possible when the Coating is applied to the surface. During la me plating, between the departure of the Coating Spray Gun and the arrival of the Coating to the surface, some of the solvent may have evaporated, slightly changing the solids by volume percentage. For most applications, this change is less significant; however, for Coating materials with fast solvent evaporation, such as vinyl and chlorinated rubber coatings, wet film thickness measurement may not be meaningful. The longer the time elapses before measurement, the less accurate the measurement will be. For Coatings with high pigment content (such as zinc-rich coatings), wet film thickness measurement may not be reliable because although the Coating contains a certain percentage of solvent, the cissing due to solvent loss may be small. Obviously, on 100% percent solids Coating, wet film thickness will be equal to dry-film thickness.

3. WFT measurement tool

Wet Film Thickness Gauges usually have a standard scoring structure. A scoring Thickness Gauge consists of two endpoints located on the same plane, with progressively deepening scoring steps in between. 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 microns. Press the instrument firmly perpendicular to the substrate into the wet Coating before pulling it out. The two endpoints will be wetting by the Coating material, as well as some of these steps. The wet film thickness is considered the last wetting step before the next adjacent, higher drying step.

conclusion

Coating thickness measurement of wet coatings has an important impact on the performance and protective ability of coatings. By accurately calculating the wet film thickness, it is possible to ensure that the Coating has an appropriate dry Coating layer after drying curing. However, when performing wet film thickness measurement, factors such as the composition of the Coating and its volume percentage must be considered. With proper measurement and calculation, accurate guidance can be provided for Coating construction to ensure the mass and performance of the coating.