Guide content
The 2% rule (give or take)
Every major polyester body filler manufacturer — Evercoat, U-Pol, US Chemical, 3M Bondo, Transtar — recommends roughly 2% cream hardener by weight. By volume, that works out to a golf-ball-sized portion of filler to a 1-inch ribbon of hardener.
Cream vs liquid hardener: not the same thing
Most body fillers ship with a cream hardener (BPO — benzoyl peroxide paste) in a small tube. This is the standard for polyester / vinyl-ester fillers like Rage, Bondo, Z-Grip, etc.
A few systems — certain glaze putties and SMC repair products — use a liquid catalyst instead. Don't substitute one for the other. Liquid catalyst in cream-hardener filler will under-cure; cream hardener in a liquid-catalyzed system can gel too fast.
What actually happens chemically
Polyester filler is a thick liquid of unsaturated polyester resin with talc, glass micro-balloons, or other fillers stirred in. When BPO peroxide is introduced, it triggers free-radical polymerization — the resin chains cross-link into a rigid solid.
More peroxide = faster cure, but also more residual peroxide trapped in the cured filler. That residual can later bloom up through topcoat as discoloration or staining (the dreaded "peroxide bleed"). That's why over-catalyzing is worse than slightly under.
Temperature changes the math
| Working temp | Adjust hardener | Pot life |
|---|---|---|
| Below 60°F | Slightly more (2.5%) | 5–7 min |
| 65–75°F | Standard (2%) | 4–5 min |
| Above 85°F | Slightly less (1.5%) | 2–3 min |
Common mistakes & fixes
Filler stays gummy / never fully hardens
Cause: Not enough hardener, hardener too old (the BPO breaks down over time — check tube date), or hardener wasn't mixed in thoroughly.
Fix: Wait 24 hours. If still soft, you have to grind it back out and re-do. Sanding gummy filler will load up paper and feather poorly.
Pinholes after sanding
Cause: Over-catalyzed (too much hardener) or under-mixed — trapped air or unincorporated hardener pockets.
Fix: Skim with finishing glaze or polyester putty over the sanded filler, then re-sand with 180–220.
Filler kicks in the pan before you can spread
Cause: Too much hardener for the temp, or the pile is too thick (heat is exothermic — the bigger the pile, the faster it kicks).
Fix: Mix smaller batches. Keep filler in a cool area before mixing.
Peroxide bleed through topcoat
Cause: Over-catalyzed filler under solvent-borne paint, OR topcoat applied before filler fully cured.
Fix: Always seal cured filler with a 2K urethane primer before basecoat. Allow full filler cure (overnight minimum on thick patches).
Pro habits
- Use a clean plastic spreader board, not cardboard — cardboard absorbs resin and changes ratio.
- Squeeze hardener from the middle of the tube, not the end — the BPO can separate. Knead the tube before each use.
- Check tube expiry. Cream hardener has a 12–18 month shelf life; old hardener is the #1 cause of under-cure.
- Don't fold the mixture — scrape and press with the spreader. Folding traps air.
Our recommendations
The body fillers most US shops re-buy weekly are listed below. Each ships with the correct ratio cream hardener; we also stock replacement BPO tubes if yours has aged out.
