Step 1. Cut out rusty metal
This truck cab corner was rusted completely through the outer sheet metal in many places along the entire rear portion of the cab. Filling the rust hole with fiberglass or body filler would not have produced a lasting repair. The rust would continue and caused the filler to bubble in a few weeks.

To make a permanent repair, the rust needs to be cut out and replaced with new metal. We cut the truck's rusty metal away with a pair of tin snips and a chisel and ground away the rust. You can use a drill and grinding stone for this step. We flanged the upper edge with a flanging tool on an air hammer to produce a recessed area (red arrow) so the new metal would fit flush with the surface of the existing body panel. That reduces the amount of filler needed to cover the seam and makes a better overall repair. Hand Flangers are also available at autobody stores that do the same thing. One could also lay the edge of the new metal patch over the truck's metal and fill the seam with body filler.

Step 2. Make new metal panel from scratch
Ready-made cab corners and patch panels that fit exactly over rusty panels are available from autobody supply stores for many vehicles. They are particularly useful for around rear fenders where there are more contours. But the bottom of this truck's cab only has a few simple contours that are easily reproduced. It's easier and faster just to make a patch from a piece of flat steel. You don't need special high-tech equipment -- here we used the jaws of a drill press vise and hammer to make the bottom body line. If you don't have a vise, you can hammer the metal over the edge of a work bench table. We checked the fit periodically until we were satisfied with the fit of our new panel.

Step 3. Fasten panel into place
The best way to fasten the new panel to the vehicle is by welding in place. That's what the pro's do. It's fast, secure, and doesn't warp the existing truck's metal. You'll need a wirefeed welder to do this. We're using an old screwdriver to hold the new metal tight against the truck. The more flush it fits, the less filler that is required. If you don't have access to a wire-feed welder, you can adhere with metal adhesive (available at bodyshop supply stores). Hold the panel in place with screws until the adhesive dries and remove the screws. Many shops use adhesive these days because it also helps seal moisture from getting into the seam. In a pinch you can fasten the panel with pop rivets, screws or braze it in place, however these aren't the best method.

Step 4. Check for straightness
The repaired surface must be relatively flat in order to make a long lasting repair using a minimal amount of filler. If there are deep recessions, more filler will be needed making it more likely that it will crack or fall off at some point down the road. Lay a straight edge on the surface and compare it to the contour on the other side of the vehicle. Lightly hammer down any high spots if necessary.