Inspection, Testing, and Quality Control
Measuring what matters
Inspection converts work into evidence. Know the core measurements and their standards: DFT per SSPC-PA 2 (gauge reading, average, and 80/20 rule), adhesion per ASTM D3359 (tape) or ASTM D4541 (pull-off), holiday detection per ASTM D5162 or NACE SP0188 (low-voltage for thin films, high-voltage for thick films), and cure testing. Know that SSPC-PA 2 requires a specified number of gauge readings per area (e.g., five readings per 100 m² for large areas) and that the 80/20 rule allows individual readings to fall below the specified DFT as long as the average meets the specification and no reading falls below 80% of the specified minimum.
QC framework
Understand the difference between quality control (applicator's own checks) and quality assurance (owner's independent inspection). Know how to document ambient conditions, surface preparation, WFT/DFT, adhesion, and holidays in a report with traceable locations and timestamps. Understand destructive vs. non-destructive testing and when destructive tests are repaired.
Traps in questions
Watch for questions that apply the 80/20 rule incorrectly - it does not permit readings below 80% of the specified minimum, and the average must still meet the specification. Another trap is using a high-voltage holiday detector on a thin-film coating, which can damage it. Also beware of assuming a single DFT reading represents a whole area.
Worked rule of thumb
Specified DFT is 300 µm. Minimum individual reading allowed is 80% × 300 = 240 µm. Readings of 250, 260, 310, 320, 330 µm average 294 µm - that fails the 300 µm average despite no reading below 240 µm. Recoat and retest.