Inspection Instruments and Test Methods
Environmental and surface checks
Measure air temperature, surface temperature, dew point, and RH with calibrated instruments. Contact thermometers and infrared thermometers are both used; remember that IR devices read surface temperature and are affected by emissivity and nearby hot or cold sources. Dew point may be measured directly (electronic meters) or derived from a sling psychrometer.
Dry film thickness
SSPC-PA 2 governs magnetic gauge measurements. A spot is the average of several gauge readings; no individual reading may fall below 80% of the specified minimum, and the spot average must meet the minimum. The number of spots scales with area - check the current edition for the exact count. Gauge types differ (Type 1 pull-off magnetic, Type 2 permanent magnet, electronic types), and all are affected by substrate curvature, edge proximity, and surface roughness. Calibrate with foils on a smooth reference surface and verify on the actual substrate where possible. Tooke-style destructive gauges cross-check readings on critical work.
Holiday and adhesion testing
Low-voltage wet sponge testers (typically battery-powered, for thinner films) and high-voltage spark testers (for thicker or conductive-substrate systems) locate discontinuities; select voltage from the manufacturer's or standard's chart so it does not damage sound film. Adhesion is assessed by tape test (ASTM D3359; lattice for thinner films, X-cut for thicker films, rated 5B best to 0B worst) or by pull-off (ASTM D4541), recording both the pressure and the failure mode (adhesive versus cohesive, and the percentage of each).
Traps
Reporting a single DFT reading as the film thickness; testing holidays in a solvent-rich or explosive atmosphere with a spark tester; and ignoring failure mode in pull-off results. Instrument calibration records must be current and traceable - an out-of-calibration gauge invalidates the survey, not just the reading.