Sanding and Surface Preparation
Purpose
Surface preparation decides whether paint sticks and stays flat. Most defects blamed on paint are really preparation defects.
Steps
- Wash and degrease with panel wipe; change rags often. Silicone and oil cause fish eyes.
- Strip or sand the old finish; featheredge repaired areas so the edge is smooth.
- Key bare steel with 80–180 grit; treat bare aluminium and galvanised steel with etch primer; prime bare steel with epoxy primer.
- Apply primer surfacer and block sand with a guide coat: 180 to shape, 320–400 to finish.
- Remove sanding dust, blow out seams, and tack cloth immediately before painting.
- Mask with the correct tape and paper; do not leave tape on fresh paint.
Guide coats and flatness
A light guide coat of contrasting aerosol shows low spots as remaining dark areas after blocking. Block with a firm pad so the paper cuts the highs and skips the lows; a DA can round off edges.
Worked example
A repaired door is primed and blocked with 320. The guide coat still shows in a low band across the middle. Sanding more with 320 there will only dish it further. Apply more primer surfacer, block with 180 on a long block, then finish at 320–400. Reasoning: blocking flattens a surface; you cannot sand a low spot flat.
Traps
- Touching prepared panels with bare hands.
- Skipping the degreaser because the panel looks clean.
- Sanding through edges and breaking through primer.
- Using worn or loaded paper that burnishes the surface.
- Not cleaning seams, so dust blows into the paint.