Coating Inspection and Quality Control
Core concepts
Inspection and QC verify that the applied system matches the specification and will perform. Candidates must understand the inspection sequence: pre-job (specification review, product data sheets, calibration), in-process (surface prep verification, environmental readings, wet-film checks), and final (DFT, holiday detection, adhesion, cure). Documentation-daily reports, batch records, calibration records-is part of the inspection product.
Measurement and testing
DFT is measured with magnetic or eddy-current gauges; take multiple readings per area and report a statistical result (average and minimum) per the specification. WFT is checked with a comb or wheel gauge during application. Holiday detection uses low-voltage (wet sponge) for thin films and high-voltage (spark) testing for thicker or immersion-service films; voltage must be set per film thickness to avoid damaging sound coating. Adhesion may be assessed by pull-off or cross-cut methods. Environmental instruments-thermometers, hygrometers, dew point meters-must be calibrated.
Traps
Common traps: reading DFT on a curved or edge surface without accounting for gauge error; using high-voltage holiday testing on thin films and creating false defects; and treating a single DFT reading as representative. Another trap is failing to calibrate gauges against a known standard before and after use. Questions may also test whether inspection is the inspector's independent judgment versus simply recording contractor claims.
Worked example / rule of thumb
Rule of thumb: take a minimum of three DFT readings per specified area, discard obvious outliers, and compare the average and the minimum against the spec. If the spec requires 8 mils minimum, an average of 9 mils with one 6-mil reading may still fail the minimum requirement-verify the acceptance criteria before signing off.