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How to Bondo Rust Holes In a Car

The fender of a car shows significant rust damage near the wheel well.

Rust is simply a fact of life. It tends to be a constant issue close to the shore, in humid areas and in the salt belt. Even a chip from a stone can give rust a foothold on your car and lead to cosmetic or structural damage. Body repair is often considered one of the most delicate and complicated car repairs, but learning how to Bondo rust holes is luckily quite simple.

While “Bondo” is the name of a 3M product, it has also become a generic term for plastic body filler, and several companies make similar stuff. The putty can be useful for filling rust holes in the most rust-prone areas of a car including quarter panels, fenders over and behind the wheels, and the bottoms of doors. However, whether or not you can Bondo a rust hole will depend on the location and severity of the rust.

To Bondo or Not to Bondo

Using body filler to reshape a curvy fender.Minor surface rust can often be sanded and painted, but major rust pitting and holes may require a little more effort and material — specifically, Bondo or some other quality body filler.

However, Bondo shouldn’t be used on every kind of rust hole. Because it can’t support any weight or protect you in a crash, you should never attempt to use Bondo for rust holes in anything structural, such as frame rails or reinforced areas on unibody vehicles (i.e. most modern cars and crossovers). Fuel tanks and exhaust systems are also a no-go because body fillers will not cure when exposed to fuel and will crack when exposed to heat.

For structural components with rust holes, welding or replacement are the only options. On the other hand, for non-structural body repairs, knowing how to Bondo rust holes can help you restore your vehicle’s appearance and prevent more rust formation.

How to Bondo Rust Holes

To get started, the body filler needs something to stick to. Rough-sanded bare metal is best, but primed metal will do. A rust converter should be used to prepare rust-pitted surfaces for body filler. Rust converter stops the oxidation process that creates rust and prevents the rust from spreading. If rust has thinned or eaten through the panel, then the Bondo needs a backup. Fiberglass sheets, metal mesh or fiberglass screen can be used.

Once you have a good surface to work with, mix body filler with a spatula or plastic scraper on a piece of cardboard. Only make what you’ll use in about ten minutes, or else the filler may cure before you are done. Using your spatula, work the filler into the surface. Try to keep the final thickness (post-sanding) less than a quarter-inch to prevent future cracking.

After 15 to 20 minutes, use coarse sandpaper (30 to 60 grit) to shape the repair and feather it to the surrounding body surface. After another 30 to 40 minutes, the surface should be ready for final sanding with 180 to 300 grit paper. After cleaning the dust from the surface, you’re ready to prime and paint. For more details, check out this write-up on the plastic body filler repair process.

There are some parts of your vehicle that may never rust, but other parts might seem like they could turn to dust right before your eyes without proper attention. Fortunately, if you catch the rust in time, you can prevent unsightly body damage or unsafe structural problems. The best approach is prevention, so be sure to take care of rust when it’s still minor with some touch-up paint or undercoating. If that rust gets to the next level, knowing how to Bondo rust holes might just save the day.

Check out all the paint and body products available on NAPA Online, or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on how to Bondo rust holes, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Image via Flickr


Benjamin Jerew View All

Ben has been taking things apart since he was 5, and putting them back together again since he was 8. After dabbling in DIY repairs at home and on the farm, he found his calling in the CGCC Automobile Repair program. After he held his ASE CMAT for 10 years, Ben decided he needed a change. Now, he writes on automotive topics across the web and around the world, including new automotive technology, transportation legislation, emissions, fuel economy and auto repair.

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