We have all used several methods of doing a patch panel repair. Make a template. Cut out the rust, make a piece to fit by scribing or copy the cut out piece. Cutting both pieces at once. All kind of methods. Here is another method.
1. Place a fabbed metal patch or donor patch or an aftermarket patch or a piece of an aftermarket patch over the rusted area. A little bigger than the rust area. Maybe 1/4 or so bigger all around. We don't want to cut too much good metal away.
2. Spot weld this piece to the existing body metal. Just a few. Spot weld the corners and spot from there, 2-3 inch spaced spots.
3. Use a thin angle grinder or die grinder to cut along the edge of the patch. Start at a corner. Go about an inch or two one way, cutting the corner spot. Then start going the other way and cut thru the first spot weld. You are NOT cutting the patch. Just the body. You are using the patch edge as a cutting guide.
4. This distance between initial spots will vary depending on the size of the patch as you will see with practice.
5. IMPORTANT: Cut at a 45 deg angle. Tilt the cutting disc top away from the patch. Do not cut with disc perpendicular.
6. Stop cutting after you have cut thru the first spot weld. You will then be able to push down on the patch and the body in that distance you just cut. This will flush up the patch to the rest of the body in that area. The patch will not fall thru if you have cut at a 45. By cutting all the way around at once or the other method of cutting both pieces all around at once will cause you to lose control of the patch and cutout. Then, as you know, will require you to hold and align the whole patch in place with magnets, tape, clamps............ This method requires no clamps, no magnets no tape, no alignment. It is a one step patch technique. Cut a distance, flush up, weld....cut a distance, flush up, weld.......
7. Cutting at the 45 prevents the patch from popping thru and you having to lift it back up and push down over and over to get a flush fit. The rusted piece will act as a spring and will begin to bring the patch down, and in place, as you go. The 45 also prevents welding the rusted piece back on. Forces it to fall away gradually. So to continue....... after you have made the first cut thru the first spot weld and have pushed the patch down gently and flushed it, spot weld again, back upstream as necessary to keep the patch flush in the area you just cut. You are then welding the patch in. Then keep going to the next spot weld and do it again.
8. Eventually you will end up with a good fitting patch and a rusted area that will fall away at the last cut.
9. If you cut too far at once you may reweld the rusted/dented piece back on again. Don't do that.
10. The other advantage to cutting at 45 is it creates a "gap" that is sort of self backed. To help prevent blow thru. You will notice that before you weld the gap, that if you take a disc and try to slide it perpendicular thru the gap, it won't go.
Of course the big rule still applies when doing final welding. NO beads, only spots, far apart, cooled with air. go slow and then go slower than that. You will see that, depending on the size and the ability of the patch to drop down as you go, will dictate how far apart and how many initial spot welds are necessary. This will take a few practice sessions like anything else. Feel free to adapt any ideas you have into this, or change anything you want to suit yourself.
Now you can "Cutt and Butt". Try it, you'll like it.
