Surface Preparation
What the exam expects
Surface preparation questions test whether you understand why each step exists, not just the sequence. Know the difference between cleaning (removing wax, silicone, road film with a dedicated pre-cleaner) and abrading (creating mechanical tooth with the correct grit). Understand substrate identification: steel, aluminum, plastic (bare, primed, or flexible), galvanized, and previously refinished panels each demand different products.
Core concepts
- Grit progression: coarser grit removes material and shape; finer grit refines scratches. Skipping grits leaves scratch marks that telegraph through topcoat.
- Primer types: etch primer for bare metal adhesion, epoxy for corrosion protection and sealing, urethane primer-surfacer for filling and sanding.
- Feather-edging and blend panels - knowing which adjacent panels must be prepped for a blend.
- Contamination control: silicone and oil cause fisheyes; fingerprints cause adhesion loss; moisture in air lines causes defects.
- Plastic prep: adhesion promoter is required on bare plastic; flexible parts need flex additive.
Traps
Questions love to swap steps: e.g., offering "sand then wash" instead of wash, then sand. Others confuse the purpose of a sealer versus a primer-surfacer, or imply that a tack cloth replaces a pre-cleaner. Watch for "always" and "never" absolutes.
Worked example
A technician is refinishing a bare aluminum hood. The correct order is: clean and degrease, abrade, re-clean, apply etch or epoxy primer within the product's recoat window, then primer-surfacer, block sand, seal, and topcoat. If a question offers epoxy over an unprepared oily surface, that is the wrong answer - adhesion requires a clean substrate first.
Rule of thumb: clean, abrade, clean again, then prime. Never sand contamination into the panel.