Inspection, Testing, and Defect Assessment
What inspectors verify
Inspection covers surface preparation, application conditions, dry film thickness (DFT), continuity (holiday testing), adhesion, cure, and appearance. Destructive or semi-destructive tests include pull-off adhesion (ASTM D4541), cathodic disbondment (ASTM G8/G95, CSA Z245.20), impact, flexibility, and indentation. Laboratory tests such as DSC or FTIR can confirm cure.
Holiday testing
Low-voltage wet-sponge testing suits thin films; high-voltage testing suits thicker coatings but can damage thin or sensitive coatings if voltage is too high. Voltage must be set per standard or manufacturer and verified on a known defect.
Defect assessment
Record location, size, and type: holiday, disbondment, mechanical damage, or blister. Repair criteria depend on coating type, depth, and whether CP can protect the exposed area. A large disbonded area can shield CP and must be removed, not merely overcoated.
Traps
Averaging DFT readings to pass a failing area; holiday testing a thin FBE at excessive voltage and creating defects; measuring DFT over blast profile incorrectly; and treating disbondment as cosmetic.
Rule of thumb
Test a representative area, not the best-looking spot, and record the lowest readings. If a defect shields CP, remove it.
Worked example
Specified DFT is 400–600 µm; readings are 380, 420, 500, and 550 µm. The average passes, but the minimum fails - the lot is nonconforming until the low area is corrected.