Coating Application and Inspection Techniques
What the candidate must know
Application inspection covers mixing, induction time, pot life, application method, and the wet-to-dry film relationship. Understand WFT (wet film thickness) measured with a comb or wheel gauge, DFT (dry film thickness) measured with magnetic or eddy-current gauges, and how volume solids link them: DFT = WFT × volume solids fraction (plus thinning effects). Know the DFT measurement rules - number and spacing of readings per area, and how to handle a reading outside range.
Other essentials: stripe coating of welds, edges, and difficult areas; overlap and pass technique for spray; holiday/pinhole detection (low- and high-voltage testers and when each applies); and curing/overcoating windows from the product data sheet. Recognize defects and their usual causes: sagging (too much applied), runs, orange peel (atomization/spray issues), pinholing (solvent or air entrapment), and mud cracking (too thick per coat).
Traps in questions
- Using WFT as the acceptance criterion when the spec requires DFT.
- Ignoring volume solids when converting WFT to DFT.
- Applying a high-voltage holiday tester to a thin or conductive coating and causing damage.
- Skipping stripe coats on edges, then blaming the topcoat for premature failure.
Rule of thumb / worked example
DFT = WFT × volume solids. If a coating has 60 percent volume solids and the spec requires 150 µm DFT, target WFT = 150 ÷ 0.60 = 250 µm. If the applicator sprays 200 µm WFT, expected DFT is about 120 µm - below spec, so adjust passes. Always confirm with gauge readings after cure rather than trusting the calculation alone.