Inspection, Quality Control, and Defect Analysis
Inspection is planned, not improvised. A coating inspection plan identifies hold points: pre-surface preparation, post-blast cleanliness and profile, pre-application environmental checks, in-process WFT and stripe inspection, and final DFT, holiday, and visual inspection. Candidates must match test methods to properties: SSPC-PA 2 for DFT, ASTM D4417 for surface profile, Bresle/ISO 8502 for soluble salts, ASTM D3359 for adhesion by tape, ASTM D4541 for pull-off adhesion, ASTM D5162 for holiday detection, and ASTM D714/D610/D4214 for blistering, rusting, and chalking.
Magnetic and eddy-current DFT gauges must be calibrated on a smooth reference plate; readings on blasted steel are influenced by profile and by the gauge's measurement principle. SSPC-PA 2 uses spot and area measurements: each spot is commonly the average of three readings; the spot average must be within 80–120% of specification and no single reading below 80%; the area average must meet specification. Defect analysis links appearance to cause: blisters from soluble salts or osmotic gradients, pinholes from solvent or air entrapment, sag from excessive WFT, dry spray from poor atomization, amine blush from moisture and incomplete cure, and mudcracking from excessive film build.
Worked example: specified DFT is 8 mils. Readings at one spot are 6.2, 6.8, and 7.0 mils. The spot average is 6.67 mils, or 83.4% of spec, which is inside 80–120%. However, 6.2 mils is 77.5% of spec and fails the no-single-reading-below-80% rule. The spot fails despite the acceptable average.