Fender rust is caused by the oxidation of a car's steel due to water exposure. Rust can be extremely corrosive and, in some instances, can eat right through a fender. Resulting fender holes can be both unsightly and structurally debilitating if the rust is allowed to continue corroding. While one solution is to replace the entire fender, this can be expensive, and sometimes tracking down a precisely matching fender can be difficult. Fixing a fender hole caused by rust can be tricky and requires some basic mechanical engineering skills.
Instructions
1. Outline the rust-infected area of the fender with a piece of chalk or other non-permanent writing material. Use a straight-edge.
2. Cut out the designated area using a power saw capable of cutting through metal.
It is likely that there will be spot welds holding the piece of fender to the car's frame or headlights. Drill out the welds and remove the rusty piece of fender.
3. Trace a replacement piece using the just-removed fender piece as a template. Cut out the replacement piece, and using a metal file, file its edges until it fits securely in the fender's new gap. This can be a long process, requiring many test fits and filing adjustments.
4. Sand off any paint from the replacement piece and from the edges of the gap. In preparation for welding, spray some primer along the edges of the panel. This prevents moisture from seeping in behind the new fender patch.
5. Put the replacement piece over the fender gap and tack in a few places using the welding torch. Test to make sure it fits securely before laying down a large amount of tack welds. The welds should never be continuous. If any area sticks out, push it flush while applying the tack.
6. Grind down the welds so the beads are as small as possible. Make sure not to grind too much in one spot or the heat could warp the replacement piece. After sufficient grinding, apply three or four coatings of Bondo. For the first coat, Bondo with short fiberglass hairs should be used because it provides extra durability for the weld joints.
7. Sand over the Bondo lightly after it has dried, and apply a primer (preferably a high build one) that will help remove scratches. The fender, free of rust holes, is now ready for painting.
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