04/23/2026
Wet & Dry Film Thickness Measurement — Red Seal Exam Breakdown
You sprayed the coating. But how do you prove it's the right thickness? The spec sheet doesn't say "looks about right." It gives you numbers in mils. The exam expects you to know how to verify them.
Here's the process step by step:
1. Read the Specification
Check the coating manufacturer's technical data sheet for the required dry film thickness in mils. Note the specified number of coats and the recommended wet film thickness per coat. DFT and WFT are related by the volume solids percentage of the coating.
2. Calculate WFT from DFT
Use the formula: WFT = DFT ÷ volume solids percentage. Example: if the spec calls for 4 mils DFT and the coating is 50% volume solids, the required WFT is 8 mils. This calculation must be done before spraying — not after.
3. Measure Wet Film Thickness
Immediately after application, press a wet film thickness gauge (notched gauge) into the wet coating perpendicular to the surface. Read the highest notch that has coating on it and the lowest notch that is clean. The WFT is between those two readings. Check multiple spots across the application area.
4. Allow Full Cure
Let the coating cure for the time specified on the data sheet before measuring DFT. Measuring too early gives false readings. Cure time depends on temperature, humidity, and coating type — epoxies may need 24-72 hours for full cure.
5. Measure Dry Film Thickness
Use a calibrated DFT gauge — magnetic pull-off type for non-ferrous substrates or electronic eddy current type for ferrous substrates. Zero the gauge on bare substrate or use certified shims for calibration. Take multiple readings in a pattern across the surface.
6. Evaluate and Document
Compare readings to the specification. Most specs allow a range — example 4-6 mils DFT. Readings below minimum require additional coats. Readings above maximum may cause cracking or adhesion issues. Document all readings on the inspection report with location references.
The exam will give you a scenario and expect you to calculate. Know the formula cold.