Body Filler
A two-part polyester putty (commonly called "Bondo") used to fill dents, smooth bodywork, and repair minor metal damage. Also: the miracle product that lets you turn a Mustang with more dents than an early-2000s pop album into something resembling a car—and the scam that hides rust when used improperly.
What 'Body Filler' Actually Means
Body filler is a thick paste made from polyester resin mixed with a hardening cream catalyst. When mixed and applied to bare metal or primer, it hardens into a sandable, shapeable surface that fills imperfections in bodywork.
Common names:
- Body filler (technical term)
- Bondo (brand name that became generic, like Kleenex)
- Plastic filler
- Putty
What it's for:
- Filling small dents and dings
- Smoothing welds after rust repair
- Shaping body lines after collision repair
- Creating smooth surface for paint
- Fixing minor bodywork imperfections
What it's NOT for:
- Structural repair (filler has no strength)
- Covering rust (rust continues under filler)
- Replacing proper metalwork (filler is for smoothing, not building)
- Thick applications (more than ½ inch is hack work)
I bought a Mustang with "recent paint job" advertised. First panel I sanded had 3/8 inch of filler over rust. The "recent paint job" was actually "recent filler and spray-can job over unrepaired rust." The filler cracked within 6 months, the rust bloomed through the paint, and I paid $6,500 to fix what should have been fixed properly in the first place. Ask me if I trust "fresh paint" listings now.
Why It Matters for Your Mustang
Body filler is necessary for smooth paint—but excessive filler hides problems:
Proper use (acceptable):
- 1/8 inch or less thickness
- Applied over solid metal or primer
- Smooths minor imperfections
- Properly mixed and applied
- Cost: $200–$1,000 for typical bodywork
Improper use (problems):
- Greater than 1/4 inch thick (will crack)
- Applied over rust (rust continues, filler fails)
- Used instead of proper metalwork (shortcuts)
- Poorly mixed (won't harden properly)
- Cost to fix later: $3,000–$10,000
The magnet test:
Hold a magnet to body panels. If it doesn't stick strongly, there's thick filler underneath. Do this BEFORE buying any classic Mustang. Thick filler = hidden rust or accident damage = run away.
Cost Impact
| Repair Type | Typical Cost (LA) | Labor Hours |
|---|---|---|
| Minor dings/dents (few small areas) | $400–$1,200 | 3–8 hours |
| Moderate bodywork (multiple panels) | $1,100–$3,200 | 8–20 hours |
| Extensive bodywork (most panels) | $2,600–$6,400 | 20–40 hours |
| Excessive filler (covering up problems) | $3,800–$10,000+ | 30–60 hours - Will fail in 1–5 years, requires redo |
*LA labor rates: $110–$140/hour for bodywork. Excessive filler done improperly will crack and fail within 1–5 years, costing $8,000–$15,000 to fix properly.
Ask me how I know these numbers.
Common Issues
Excessive Thickness
More than 1/4 inch thick will crack (maximum acceptable: 1/4 inch)
Filler Over Rust
Rust continues growing under filler, filler fails in 6 months to 3 years
Poor Mixing
Too much/too little hardener = won't cure properly or will crack
Used Instead of Metalwork
Filler is for smoothing, not building—needs proper metal repair first
Painting Directly on Filler
Filler is porous, must be primed before painting (absorbs paint solvents)
See This in Action
- Mustang Bodywork Cost Guide
Detailed body filler vs. metalwork cost comparisons and when each approach makes sense
Want to Learn More?
Download the Mustang Restoration Starter Kit (LA Edition) for:
- Complete terminology reference guide
- Cost estimation worksheets
- Pre-purchase inspection checklist
- Shop interview questions
- Project timeline planning tools
No upsells. No bait-and-switch. Just the information Dorian wishes he'd had before he bought his first project car.