Body Filler Application
Products
Polyester body filler is the standard; finishing glaze fills pinholes and sanding scratches; fibreglass filler bridges larger gaps; aluminium-filled filler suits some bare-metal uses. Filler shapes a panel; it is not structural, and it should be thin.
Application rules
- Prepare the surface: bare steel keyed with 36–80 grit, clean and dry. Filler does not bond to rust, wax, silicone or glossy paint.
- Mix on a clean board at the correct ratio, usually about 2% hardener by weight. Too little leaves a sticky, weak film; too much makes it brittle and pinhole.
- Spread with a flexible plastic spreader, pressing into the surface, in thin coats. Build to a maximum around 3–6 mm.
- Shape before full cure with a cheese grater or coarse paper, then finish with 80, 120, 180 and 320 grit, featheredged into the surrounding paint.
- Finish with glaze and sand to 320–400 before primer.
Worked example
300 g of filler needs about 6 g of hardener at 2%. If you add a 20 g lump, the filler kicks in a minute, traps air and sets brittle, and you will chase pinholes after primer. Mix smaller batches and measure by weight or use the marked hardener strip.
Traps
- Mixing on cardboard, which absorbs resin.
- Filling over rust, paint edges or silicone.
- Thick coats that shrink and print through.
- Sanding with 36 grit and painting over the scratches.
- Leaving pinholes that blow out in the bake.