Coating Application and Inspection
Application variables
Understand how application method shapes film quality. Brush and roller suit stripe coats and small areas; conventional air spray, airless spray, and plural-component spray differ in atomization, transfer efficiency, and suitability for high-viscosity or fast-curing materials. Know that mixing ratio, induction time, pot life, and thinning affect cure and performance - exceeding pot life or adding wrong thinner are common failure causes.
Film thickness and defects
Distinguish wet film thickness (WFT) from dry film thickness (DFT) and use the relationship DFT = WFT × volume solids fraction. Traps appear when a candidate forgets to convert volume solids from a percentage. Common defects to recognize: runs/sags (too much material), orange peel (atomization/viscosity), holidays and pinholes (incomplete coverage, especially at edges), mud cracking (excessive DFT), and amine blush (epoxy in cool/humid conditions).
Inspection instruments and methods
Know non-destructive DFT gauges (magnetic/eddy current), holiday detectors (low-voltage wet sponge vs. high-voltage spark testing, chosen by film thickness and substrate), adhesion tests (pull-off, cross-cut), and environmental instruments (dew point meters, hygrometers, surface thermometers). Understand destructive vs. non-destructive testing and when each applies.
Worked example: A coating with 60% volume solids applied at 8 mils WFT yields 8 × 0.60 = 4.8 mils DFT. If spec requires 5 mils minimum, the application is short and must be corrected.