Coatings Inspection: Verifying the Specification
Core concepts
Inspection confirms that what was specified was actually delivered. The inspector's toolkit spans pre-job, in-process, and final checks. Before application, verify surface preparation (blast grade, cleanliness, anchor profile) and environmental conditions (ambient and substrate temperature, relative humidity, dew point). During application, monitor wet film thickness, stripe coats, and pot life. After cure, measure dry film thickness (DFT), adhesion, and continuity (holiday detection).
Know the instruments and their limits. Magnetic/electromagnetic gauges measure DFT on ferrous substrates; eddy-current gauges on non-ferrous; ultrasonic gauges for non-destructive measurement on various substrates. Each has calibration requirements - verify against known standards and check on the actual substrate. Holiday detectors use low voltage for thin films and high voltage for thicker or immersion service; the voltage setting matters.
Understand DFT statistics: a single high or low reading rarely condemns a job. Specifications typically define a minimum, maximum, and average, plus the number of readings per area. Adhesion testing (pull-off, knife) is destructive and typically done on a representative basis.
Traps
Questions test whether you know which instrument applies to which substrate and whether calibration was verified. Watch for "average passes but minimum fails" scenarios - both criteria usually apply. Also distinguish inspection (verification against spec) from testing (measuring a property).
Rule of thumb
Calibrate before and after, and measure on the same substrate and curvature as the work. A gauge reading you cannot defend is not a measurement.
Worked example
A specification requires 5–8 mils DFT, average ≥6 mils, with five readings per 100 ft². Readings of 4, 6, 7, 7, 8 give an average of 6.4 mils - the average passes, but the 4-mil reading is below the minimum and triggers corrective action. The average alone does not save the job.