Why it is measured
Dry film thickness is the acceptance criterion in most specifications, but it can only be measured once the coat has dried, when correcting it is harder. Wet film thickness gives the applicator feedback in real time. If the reading is low, the painter adds another pass while the film is still open; if it is high, the risk of sags, runs, solvent entrapment or cracking can be addressed immediately.
How the target is set
The target WFT comes from the specified dry film and the volume solids on the technical data sheet:
- target WFT equals target dry film thickness multiplied by 100, divided by volume solids;
- when thinner is added, the wet film must be increased to compensate, according to the thinning allowed by the manufacturer.
The data sheet usually lists recommended wet and dry thicknesses per coat, which should be used as the reference.
How it is measured
| Method | Principle | Notes |
|---|---|---|
| Notch (comb) gauge | Teeth of stepped heights are pressed into the wet film; the reading lies between the last wet tooth and the first dry one | Most common field tool, ASTM D4414 and ISO 2808 |
| Wheel gauge | An eccentric wheel is rolled through the film | Gives a reading on smooth, flat areas |
Good practice for the notch gauge:
- press it perpendicular to the surface, immediately after application;
- take readings on flat areas and away from edges, then repeat across the piece;
- clean the gauge after each reading;
- record the readings when the inspection plan requires it.
Limits
WFT is a control tool, not an acceptance test. Fast-drying coatings can lose solvent between application and reading, and rough or profiled surfaces make readings less precise. Zinc-rich and other highly pigmented coatings may also be difficult to read. Final acceptance remains the dry film measurement per SSPC-PA 2 or ISO 19840, followed where specified by a holiday test.
Readings taken during application can also be compared with later dry film measurements to confirm the volume solids assumed in planning.
Our coating inspection service checks film build at each stage, wet and dry.