Topcoats, Clearcoats, and Finishing Defects
What this domain tests
Basecoat, metallic control, clearcoat application, curing, defect identification, and polishing.
Core rules
- Apply basecoat in thin, even coats to hiding. Do not sand basecoat; it is not a surfacer.
- For metallics, use a drop coat or orientation coat: lower pressure, greater distance, and a light pass to align the metallic. Keep the gun parallel to the panel.
- Clearcoat is a 2K product; mix accurately, use the correct reducer, and apply within pot life. Two medium coats are typical, with flash-off between.
- Maintain a wet edge to avoid dry spray and mottling. Avoid excessive air movement over the wet film.
- Do not polish fresh clearcoat until it has cured as specified; polishing too early dulls or mars it.
- Identify defects by cause, not by name: runs are gravity and too much material; orange peel is viscosity, pressure, distance, or temperature; solvent pop is trapped solvent; fish eyes are contamination.
Worked example
Clearcoat dries with fine bumps and a pebbled surface. The gun was set at low pressure, the clear was reduced too little, and the booth was cool. Reasoning: high viscosity plus low atomisation gives poor flow; cool air slows levelling. Correct method: use the specified reducer, set inlet pressure and fan, warm the booth to the sheet range, apply two wet coats with flash-off, and allow cure. If film thickness allows, wet-sand with 1500 to 2000 and polish.
Traps
- Sanding metallic basecoat and disturbing orientation.
- Over-thinning clearcoat and causing runs or die-back.
- Polishing before the clear has hardened.
- Calling every small crater a fish eye when it may be dust or solvent pop.