Wet Film Coating Thickness
Wet Film Coating Thickness measurement is an early method of determining whether the Dry Film Coating Thickness will be correct. It is important that a coating is applied to the correct thickness. Applying too much wet coating will not only waste time and money, but there is also a possibility of the coating cracking during the curing process. Too little coating and there is a chance that the substrate will not be sufficiently covered.
To control process variables, it is often desirable to measure whilst the film is still wet. Wet film measurements are also useful for systems where the dry film thickness can only be measured destructively.
When applying a liquid coating, by measuring the uncured film thickness, it is possible to determine the eventual dry film thickness. Applying too much coating wastes time and materials. It can also affect the performance and finish of the product. Too much wet film can cause the coating to crack as it cures; too little coating increases the risk that the substrate will not be sufficiently protected, leading to rust spots.
Wet film combs are available in a variety of materials including stainless steel, aluminium and plastic and with various measurement scales ranging from 20µm to 3,000µm.
Using a wet film comb:
- Place a comb perpendicular to and touching the substrate.
- Hold the comb in position and wait a few seconds until the teeth are wet.
- Remove the comb from the film.
- The wet film thickness lies between the biggest value ‘coated’ or ‘wet’ tooth and the smallest value ‘uncoated’ or ‘dry’ tooth.
Wet film wheels are high precision, accurate and easy to use instruments consisting of a set of three wheels, with measurement ranges from 0-25μm to 0-3,000μm.
Using a wet film wheel:
- Roll the wheel through a wet coating.
- The centre wheel eventually touches the film – this point on the scale indicates the thickness.
- When the volume to solids ratio of the coating is known, generally found on a product data sheet, the wet film thickness can be used to predict the dry film thickness.
- Roll from maximum to minimum to avoid a false reading caused by surface tension.