Surface Preparation and Substrate Assessment
What this domain tests
Substrate identification, cleaning, damage assessment, sanding, feathering, bare-metal treatment, and adhesion promotion before primer. The exam expects a logical preparation sequence, not random sanding.
Core rules
- Identify the substrate first: steel, aluminium, galvanised steel, plastic, OEM e-coat, or old refinish. Each needs a different primer or adhesion promoter.
- Wash with water and mild detergent to remove road film, then use a wax-and-grease remover. Do not use lacquer thinner as a final wipe; it can leave residue.
- Sand with a grit progression: 80 for shaping filler, 180 to 240 before primer, and 400 to 600 before basecoat. Jumping grits leaves scratches that show through.
- Feather edges back to a smooth transition; a sharp edge shows as a ring.
- Bare steel must be cleaned and coated promptly with epoxy or etch primer. Aluminium should be abraded and primed; galvanised steel needs a suitable primer, not ordinary etch.
- Plastics need cleaning, sanding, and a plastic adhesion promoter; flexible parts need flex additive in the topcoat system.
- Use a guide coat to find low spots and sanding scratches.
Worked example
A door has a shallow dent and old paint around it. Plan the preparation. Reasoning: strip the repair area to bare metal, hammer and dolly the dent, grind a 75 to 100 mm featheredge, sand the surrounding paint with 180, clean twice, prime bare metal, apply filler only where needed, sand filler with 80 then 180, prime-surfacer, guide-coat, and finish with 400 to 600 before base. The featheredge prevents a visible edge.
Traps
- Silicone contamination causes fish eyes; wipe and change rags.
- Sanding through OEM e-coat on galvanised steel exposes zinc and changes adhesion.
- Steel wool on aluminium embeds iron particles and causes corrosion.