This week’s tech tip addresses film thickness measurement and control — one of the most important quality parameters in polyurea application, and one where technique variations between applicators can produce dramatically different outcomes.
Why Film Thickness Control Matters
Film thickness directly determines coating performance. Under-thickness creates vulnerability — insufficient barrier, reduced abrasion resistance, potential pinholes. Over-thickness wastes expensive material and can create application problems like mud-cracking in certain formulations. Specified thickness for a given application represents an engineering decision, not an arbitrary number.
For high-stakes applications like secondary containment liners and potable water tanks, achieving specified thickness across the entire application is not optional — it’s a regulatory compliance requirement.
Measurement Methods
Wet Film Gauge
A notched wet film gauge (comb gauge) measures coating thickness while still wet. For polyurea, this is challenging because the gel time is so short. However, for slower-setting hybrid systems, wet film measurement can provide real-time feedback. The technique: immediately after spraying a test pass on a separate surface, press the comb into the wet coating. The highest tooth that’s wet indicates the film thickness.
Dry Film Thickness Gauge (DFT)
Magnetic induction gauges measure DFT on steel substrates. On concrete and other non-ferrous substrates, eddy current or ultrasonic gauges are used. Ultrasonic gauges are the most versatile and are becoming standard in professional polyurea applications. Take multiple readings per area and record the data — good documentation protects you and demonstrates quality.
Destructive Testing
For verification purposes, drill-gauge (a calibrated cutting tool that cuts through the coating to measure thickness) provides absolute confirmation but damages the coating. This method is used when non-destructive measurements are disputed or when calibration verification is needed.
Pass Strategy for Consistent Thickness
Achieving consistent film thickness requires a systematic pass strategy:
- Establish your overlap: For most polyurea guns, a 50% overlap between passes gives the most even build. Too much overlap creates ridges; too little leaves thin areas between passes.
- Consistent gun speed: Faster gun movement = thinner film. Develop a consistent arm speed and test it on a calibration panel before starting a project.
- Consistent gun distance: Most polyurea systems spray at 20–30cm from the substrate. Varying this distance changes both film thickness and spray pattern width.
- Multiple coats for critical applications: For specified thicknesses above 50 mils, applying in multiple passes (typically 2–3) achieves more consistent thickness than a single heavy pass and allows intermediate inspection.
Calibration Panels
Every project should start with a calibration panel — a piece of steel or primed surface where you establish your actual output at your working settings. Spray your calibration panel, measure DFT, and adjust technique (speed, distance, overlap) before spraying the actual project surface. This ten-minute investment prevents thickness non-conformances on the actual work.
For more on establishing quality assurance protocols, see the CPCA Surface Preparation and QA Field Guide.
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