What is "film thickness"?

Film thickness is the depth of the applied coating. Film thickness can be roughly divided into:

wet film thickness

Wet Film Thickness (WFT) is the thickness of a wet paint or any liquid-based coating film. Wet thickness peaks immediately after coating application. It is reduced once the volatiles and solvents in the paint film evaporate. When measuring wet film thickness, it is necessary to determine the amount of material that needs to be applied to achieve a specific dry film thickness, which provides better protection from damage, wear and corrosion.

Dry film thickness

Dry Film Thickness (DFT) is the thickness of a coating measured on top of the substrate after the coating has dried. This can consist of a single layer or multiple layers. Measure the DFT of the cured coating (after the coating has dried). The thickness of the coating depends on the application and the type of process employed. Proper thickness should be determined by recommended coating system parameters.

Measuring the wet film thickness of a paint or coating applied to a material is extremely beneficial in determining the proper dry coating thickness. Almost all better performing protective coatings should meet specific requirements for dry film thickness. Therefore, there is a direct relationship between wet film and dry film thickness.

The ratio of dry film to wet film is determined by the volume of volatiles present in the paint applied and the degree of dilution allowed. In flat coatings, the wet film thickness is usually lower than the dry film thickness. Wet film thickness measurement at the point of application is preferable as it allows film adjustments and corrections to be made at the point of application. Calibration after the film has been chemically cured or dried takes a lot of time and labor and can lead to film fouling. Additionally, it can cause issues such as reduced coating integrity and adhesion.

To measure wet film thickness, various gauges are used. Common ones are:

Notched Gauge - For uneven surfaces such as rough concrete blocks. They are also a sensible choice in the field and workshop, especially for determining the estimated wet film thickness of commercial items whose shape and size are not suitable for other specification types (such as thin edge, oval, etc.).

Pfund and interchemical gauges - These gauges are suitable for use on uniform surfaces and provide sensitive and accurate wet film thickness measurements compared to notched gauges.

DFT is a parameter considered during quality control or inspection. The durability of the coating is a function of the DFT, while the thickness of the individual coatings in the coating system and the reported DFT of the entire system specify the durability of the coating.