Choose the wrong body filler and it’ll crack, delaminate, or absorb moisture within a year. There are three main resin types used in automotive body filler, and each has a specific job. Here’s how to know which one your repair needs.
The Three Main Body Filler Resins
1. Polyester (Standard “Bondo”) — For 95% of Cosmetic Repairs
Polyester filler is what most people call “Bondo” (a brand name). It’s a two-part filler with a cream hardener (BPO). Cures in 15-30 minutes, sands easily, adheres well to properly prepped steel.
Use for: Dents, small rust patches after treatment, panel skimming, feathering edges.
Don’t use for: Bare aluminum (won’t bond long-term), over rust that hasn’t been treated, structural repairs, panels exposed to constant moisture.
Common brands: 3M Bondo, Evercoat Rage, U.S. Chemical Ever-Ply.
Cost: $18-45 per gallon. Compare on Amazon.
2. Epoxy — For Aluminum, Fiberglass, and Anti-Corrosion
Epoxy fillers use two parts (resin + hardener) with different chemistry from polyester. Longer cure time (2-8 hours), harder to sand, but bonds to nearly anything: bare steel, aluminum, fiberglass, plastic. Also acts as a moisture barrier — this is what makes it invaluable for panels near coastal environments.
Use for: Repairs on aluminum panels (F-150, Tesla, modern German cars), fiberglass boats and Corvettes, rust prevention layers under polyester filler, marine/coastal environments.
Don’t use for: Quick weekend fixes (slow cure), thick fills (heat management issues).
Common brands: West System G/flex, System Three, Evercoat Metal Glaze.
3. Polyester Reinforced with Fiberglass (“Kitty Hair” / Long-Strand)
Standard polyester resin mixed with chopped fiberglass strands. Much stronger than plain polyester — used for filling holes, spanning gaps, and building up structural areas before smoothing with regular filler.
Use for: Rust perforation holes, gaps in panel work, first layer over sheet metal patch repairs, over-fill on rocker panels.
Don’t use for: Smooth finish work — the fibers make final surface impossible without top layers of regular polyester.
Common brands: Evercoat Tiger Hair, 3M Bondo Reinforced Filler.
What About Metal-to-Metal Filler?
Aluminum-filled polyester (like Evercoat Metal-2-Metal or 3M Metallic) contains aluminum particles for extra strength. Use it as a first layer over patch panels, weld seams, or areas that flex. Standard polyester goes on top for finish smoothing.
How to Choose Correctly
| Repair Situation | Best Filler |
|---|---|
| Small dent on painted steel | Standard polyester |
| Dent on aluminum panel (F-150, Tesla) | Epoxy (first layer), then polyester |
| Rust hole (up to 1″ diameter) | Kitty hair, then polyester finish |
| Rust hole (larger than 1″) | Metal patch first, then filler |
| Fiberglass panel repair | Epoxy resin |
| Coastal environment (salt exposure) | Epoxy sealer under all filler |
| Show-car finish work | Polyester glazing putty (final smoothing) |
The Correct Application Sequence
- Bare metal: Grind to bright metal, treat any rust with converter, apply epoxy primer if on aluminum or in corrosive environments.
- First fill layer: Metal-to-metal or kitty hair for structural fills, standard polyester for cosmetic.
- Skim coat: Standard polyester filler, thin layer to shape.
- Sand: 80 grit rough, 180 grit shaping, 320 grit finish.
- Glazing putty: Fine polyester glaze fills 80-320 grit sanding marks.
- Final sand: 400-600 grit.
- Primer and paint.
Common Filler Mistakes
- Applying too thick: Filler cracks if applied more than 1/4″ thick. Do multiple thin layers.
- Wrong hardener ratio: Too much = brittle and cracked. Too little = never fully cures.
- Applying over rust: Rust continues underneath and lifts the filler within a year.
- Applying to bare aluminum with polyester: Will delaminate. Epoxy primer required first.
- Sanding too aggressively when still curing: Filler tears out of the low spots. Wait full cure time.
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