Inspection Instruments
Match the instrument to the substrate
Dry film thickness gauges divide into Type I (magnetic pull-off, permanent magnet) and Type II (electronic, magnetic induction or eddy current). Magnetic methods require a ferrous substrate; on aluminum or galvanized steel you need eddy-current or ultrasonic gauges. A reading taken over a grit profile includes part of that profile, so verify the gauge on smooth uncoated steel of the same type and keep the same reference for the coated reading.
SSPC-PA 2 in practice
For each spot, take three gauge readings and average them - that average is one spot measurement. Take five spots per 100 square feet across a 10-foot grid and record the highest and lowest. Acceptance typically requires the average of all spot readings to meet or exceed the specification and no individual reading below 80% of specification; the specification may also set a maximum.
Worked example: specification is 5.0 mils minimum. Spot 1 readings 4.2, 4.6, 5.0 average 4.6 mils. 80% of 5.0 = 4.0, so 4.6 passes the lower limit. But the overall average of all spots must still reach 5.0 - if the five spot averages are 4.6, 5.2, 4.8, 5.1 and 5.3, the grand average is 5.0 and the job passes.
Other field instruments
Wet film combs for immediate checks; low-voltage wet-sponge holiday detectors for thin films and high-voltage spark testers for thicker or immersion-service films; replica tape and comparators for profile; sling psychrometer, electronic dew-point meter, and surface thermometer for environment; pull-off adhesion testers and knife or cross-cut tools for adhesion; solvent rub for cure of specific coatings.
Trap: readings rounded up, or a single low reading "balanced" by a high one. PA 2 averages spots, not individual readings scattered across a wall.
Rule of thumb: verify gauge accuracy against a known reference before and after each survey, and write the verification down.
Testing and Documentation
What gets tested, and when
Testing falls into four groups: thickness, continuity, adhesion, and cure or appearance. Thickness uses the DFT gauge per SSPC-PA 2. Continuity uses holiday detection - low-voltage wet sponge for thin films, high-voltage spark testing for thicker films - with voltage set to the film thickness, not simply raised until a spark appears. Adhesion uses pull-off testing (ASTM D4541) or cross-cut and knife testing (ASTM D3359), and every destructive test leaves a repair that must be documented and recoated. Cure checks include solvent rub for specific coatings such as inorganic zinc, pencil hardness, and gel time for plural-component materials.
Trap: questions implying a single test proves quality. A coating can pass DFT and fail adhesion; a holiday-free lining can still fail because surface salts were never measured.
Documentation is the deliverable
The inspector's records are the evidence. Typical documents include the project specification and product data sheets, daily inspection reports (weather, temperatures, dew point, RH, substrate condition, materials and batch numbers, application method, film thickness, deviations), nonconformance reports, calibration records, dated photographs, and the manufacturer's technical data. Write observations contemporaneously - a report reconstructed from memory a week later is weak evidence in a dispute. Keep the daily report objective: "surface temperature 48°F, dew point 46°F" rather than "conditions looked marginal."
Worked example: pull-off testing returns 600 psi where the specification requires a minimum of 1,000 psi. The correct response is not to retest until a favorable number appears and not to average it away: record the result at the location, issue a nonconformance report, and escalate to the owner's representative for disposition.
Rule of thumb: if it isn't written down with a number, a date, and a location, it didn't happen.