How to Repair a Rusty Fender

Here, Paradis trimmed the top cut with only a tiny gap between the patch panel and the fender, which can easily be filled with a MIG welder. The same process was slowly accomplished on the two vertical edges. Note that we are making what is called a butt-weld connection as opposed to an overlap weld. Overlapping is easier but creates a seam that can trap moisture and eventually cause more rust.

Paradis used masking tape to indicate the edge of the trim line on this vertical edge. A pencil line would also work. Paradis also emphasized proper use of the angle sander. These units spin clockwise, so using the trailing edge of the disc will ensure the sparks fly off the edge of the metal and will prevent catching an edge with the sanding disc, perhaps causing an injury.

With all three edges trimmed, Paradis then cut out the rusted lower edge that tied the bottom of the fender together. He waited until the patch was precisely trimmed to cut the lower portion. This way, the patch fit better.

Before tacking the patch in place, we blasted the rust off the inside reinforcement panel to ensure a clean surface and also to help with restoring that piece, which we'll do later. We also then cleaned it with Metal Prep and sealed it with a weld-through primer.

Paradis then clamped the patch panel in place with several Vise-Grips to position it properly before tack welding with a MIG welder.

During the tack-welding process, Paradis used his hands to force the patch panel level with the original fender to produce the best fit. He placed dozens of tacks throughout the entire area to position the new panel accurately.

The finish-welding isn't much different from just more numerous tacks. Paradis applied very brief, tacklike beads of weld and worked around the entire patch in different locations to prevent building heat into the fender that could cause warping. Again, work slowly and allow the panel to be cool to the touch in between welds.

After the welding was completed, Paradis used the angle sander with a 36-grit disc to grind down the welds. Be careful not to grind off so much that the weld is destroyed.

This could pass for a finished fender repair, although we discovered a couple of areas after grinding that need some spots of weld to fill. Next will come a thin spread of Bondo to complete the repair, and then it will be ready for primer and finish-sanding. But first we have to repair that rusted inner panel. Then it will be time to attack the other fender and the doors.