Surface Preparation and Substrate Identification
Surface Preparation and Substrate Identification tests whether you can recognize what you are coating and prepare it so the refinish system adheres and does not react. Substrates include bare steel, aluminum, galvanized steel, plastic, OEM e-coat, cured OEM paint, and previous refinish.
Core rules
- Wash with wax and grease remover before sanding; sanding drives contamination into scratches.
- Identify plastic type. TPO and PP often need a dedicated adhesion promoter; rigid ABS may only need cleaning and scuffing. Follow the product system.
- Treat bare steel with the specified metal treatment or epoxy primer; do not leave bare steel exposed.
- Aluminum needs cleaning and the specified primer; avoid steel wool that leaves ferrous particles.
- Galvanized steel requires removal of white corrosion and a compatible primer; acid-based treatments can react with zinc.
- Feather edges, sand with the recommended grit progression, remove sanding residue, and use a tack cloth before sealer.
- Do not sand through OEM e-coat to bare metal unless the repair requires it; break-through changes primer needs.
Worked example
A TPO bumper cover has a scuff through the factory finish. Clean with plastic cleaner, sand the damaged area with 320-400 grit, featheredge, blow out dust, apply a TPO-specific adhesion promoter, then flexible primer and topcoat. Reason: TPO has low surface energy; without the promoter, topcoat may peel even if the surface looks sanded.
Traps
Using a magnet to prove aluminum, skipping a plastic cleaner, sanding a flexible part with coarse grit and no flex additive, and leaving sanding dust under sealer.