Body filler is straightforward to use badly and takes a little discipline to use well. Almost every common failure — pinholes, staining through the topcoat, filler lifting at the edges, a repair that shows up months later — comes from surface preparation, the hardener ratio, or applying too much material at once.
Preparing the surface
Filler needs to key to sound, clean, abraded metal or a suitable primed surface. Not to paint, not to rust, not to a greasy panel.
- Take the repair area back to bare, sound metal and feather the surrounding paint edges.
- Abrade the bare metal to give a key — a coarse grit gives filler something to grip.
- Degrease with panel wipe and a clean cloth immediately before filling. Do this after sanding, not before.
- Make sure the area is completely dry.
Filler applied over rust will fail. There is no shortcut here — the corrosion has to come out.
Mixing
Two-part fillers cure by chemical reaction with a hardener, and the ratio matters. Always follow the ratio on the tin or the product's technical data sheet — it varies between products and, for some, with temperature.
- Too little hardener: the filler stays soft, may never cure fully, and can stain through paint later.
- Too much hardener: it goes off before you can spread it, becomes brittle, and the excess peroxide can cause discolouration through the topcoat.
Mix by folding rather than whipping. Stirring in a circular motion drags air in, and every trapped bubble becomes a pinhole you'll be filling again later. Mix on a clean non-porous board — cardboard absorbs resin and shifts the ratio.
Only mix what you can apply within the working time. On a warm day that may be a couple of minutes.
Applying
Build up in thin layers. A first tight coat pressed firmly into the abraded metal wets out the surface and gives the bond its strength. Subsequent layers build the shape.
Aim slightly proud of the final contour so you have material to sand back — but resist filling the whole dent in one pass. Thick applications cure unevenly, shrink more, and trap air.
Let each layer cure properly before the next.
Sanding back
Shape with a coarser grit and refine progressively. A typical progression runs from around P80 for shaping, through P180, to around P320 before primer — but follow the filler and primer data sheets, since a primer's fill capability determines how fine you need to go.
- Use a sanding block or board on flat panels. Fingers create hollows.
- Work in a cross-hatch pattern to reveal high and low spots.
- Check by feel with a clean, flat palm — your hand finds imperfections your eye misses.
- Net abrasives such as Mirka Abranet extract dust through the mesh, which keeps the abrasive cutting and cuts clogging significantly on filler.
Finishing and pinholes
Pinholes will almost always appear somewhere. Rather than reaching for more body filler, skim with a fine finishing filler or stopper — U-POL Dolphin Glaze is designed for exactly this — then sand back and prime.
Mask up before primer with a proper automotive masking tape, and use a high build primer to fill remaining sanding scratches.
Choosing a filler
Lightweight fillers such as U-POL Fantastic sand easily and suit medium-depth repairs. Fine finishing fillers are for the last skim, not for building shape. Browse the full paint and bodyshop range.
