Body Filler, Preparation and Surface Finishing
Body Filler, Preparation and Surface Finishing
This domain covers the steps that make a repair invisible: substrate preparation, filler mixing and shaping, priming, blocking and guide coating.
Core rules
- Prepare by washing, degreasing and stripping to bare metal; feather-edge the paint edge so the repair blends.
- Apply epoxy primer to bare steel before filler where specified; filler should not be used over rust or directly over etch primer unless the product data says so.
- Polyester filler is mixed with MEKP hardener at about 2–3% by weight; too much hardener makes it brittle and fast-setting.
- Apply thin coats, shape with a body file or 80-grit, then refine with 180-grit on a block.
- Prime, apply a guide coat, and block-sand with 320–400 grit to reveal low spots.
- Clean with a tack cloth before topcoat.
Worked example
A dent is filled and sanded but the repair shows a ring under the primer. Reasoning: the filler edge was not feathered and the guide coat was sanded too aggressively at the edge. Re-feather the edge, re-prime, apply a thin guide coat, and block-sand until the guide coat disappears evenly.
Traps
Using too much hardener, applying filler too thick, sanding with the fingers instead of a block, and contaminating the surface with silicone or oil. Also, sanding through a paint edge.