Bondo is a plastic filler used to patch holes and fill dents in a vehicle's sheet metal. The material has the consistency of thick putty, and comes with a tube of hardener. The putty will stay soft and malleable until you add the hardener, and how fast it hardens depends upon the amount you put in. The hardener is dark in color, so you can determine how much (and how well dispersed) it is in the light-colored* putty.

To prepare a dent for Bondo, you first pull the dent out to just below the level of the surrounding surface, to create a shallow hollow for the putty. If there is a small hole or two (from a slide hammer, for example, or a corner of whatever caused the dent, don't worry, the bondo will fill it in. Sand the entire area, including a healthy portion of the area around the dent, to bright metal. If the hole is large, you need to get some fiberglass cloth to fill the gap. The bondo will fill in the spaces in the cloth, creating a fiberglass-reinforced patch.

Once the Bondo is dry, simply sand it flat and paint. If you take your time and be careful to do a good job, the patch will blend into the area to where only a close examination with a magnet will tell where it is.

*allseeingeye points out that "real" Bondo is pink. There are "off-brand" versions that work (nearly) as well, but are gray or pasty (pardon the pun) white.

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