Bondo is the trademarked name of a polyester resin material used to repair automobile bodies (and sometimes wood and other materials). It was developed as a replacement for lead products used for years to fill dents, dings and other imperfections in automobile bodies. Lead is toxic; Bondo also contains toxic chemicals but is much safer than lead. It also is easier for a car owner to use; lead requires special training and equipment. While similar products are sold in auto parts stores, Bondo has become almost a generic name for body filler.
Instructions
1. Prepare the surface by knocking out or scraping away any visible rust, holes or imperfections; use a knife, paint scraper or steel brush to scrape off as much rust as possible and get the surface as smooth as possible. Sand the area with medium-grit sandpaper, by hand or with a power sander, until the area is level. Run your hand over it and if it feels smooth, sand again with fine sandpaper until bare metal shows. Wipe the area clean with a damp cloth.
2. Put a small pile of Bondo on a waxed cardboard or plastic sheet, add a strip of hardener (check the recommendations for amounts; too much hardener will make Bondo unworkable, too little will not let it harden properly). Mix the two parts with a putty knife until the material is a uniform color.
3. Spread Bondo on the repair area with a putty knife; use a wide knife for big areas. Smooth it as evenly as possible. Avoid leaving ridges or valleys in the Bondo. Work in small sections when repairing a big area and try to fill all the spaces at one time; repeated applications may create problems because of different hardening. Back up rust holes which go clear through the metal with duct tape on the inside of the fender or other area; this will keep Bondo mostly on the surface.
4. Let the Bondo harden completely; this will only take a few minutes. Smooth big bumps with a utility knife, metal file or planer/shaper, then begin to sand the area smooth. Start with medium-grit sandpaper, by hand or with a power sander, and work the repair as smooth as possible, then switch to finer sandpaper and finish with very fine sandpaper until the area is perfectly smooth and level.
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