This tool is used to measure the thickness of a wet paint film as it is being applied to a surface. It consists of a flat, rectangular metal or plastic blade with notches or markings on the side that is placed onto the wet paint film. When the gauge is lifted from the wet paint film, the amount of paint that has adhered to the blade is measured in mils (thousandths of an inch) or microns.

Measuring the wet film thickness is important in ensuring that the coating is applied to the correct thickness, as too thin or too thick coatings can lead to issues with adhesion, durability, and appearance. Wet film thickness is also used to calculate the dry film thickness, which is the thickness of the coating after it has dried or cured.