My 74 C10 has some rust issues. While the truck is structurally sound, there are a couple of areas on the outer skin that have completely rotted through. Yesterday, I started tackling these issues, and I figured I'd write up a little step by step guide to repairing rust.
*Disclaimer: I am by no means an expert sheet metal guy. Hell, this is the first time I'd done this. I do though have a good amount of sheet metal/fabrication/welding/general shop experience, so a lot of these actions I'd done before, just not in this setting. The finished panel is by no means a show piece, and is more a function over form type piece.
Things you will need to tackle the job:
1) Replacement patch panel/sheet metal to fill the gap
2) Some sort of sheet metal cutting tools. I used a pneumatic cut-off wheel
3) Welder. I used MIG, but mostly because I was too lazy to get the TIG out.
Ok. So step one is going to be to assess the damage, and strategize your plan of attack. This involved wire brushing the whole area and seeing where the rust ended and solid sheet metal began. I chose to keep it simple, and just replace a large square of rust. I marked off the area with tape, and set about cutting.
Step 2: Cut out said area. Try to cut as straight of lines as possible, because you'll have to match the line you cut out of the panel with the cut you make in the patch panel.
While I was at it, once the old metal was cut away, I wire brushed the inner panel, sprayed all the debris out, and shot some black primer over it. Just a little protection from future water somehow seeping into it.
So the next step after that is going to be prepping the patch panel. Now, this is where I'm not sure if the process I did was kosher, but it made sense to me. I think If I were replacing the whole area and not cutting a section out of the panel, it may have been easier than trying to measure and cut a very curved patch panel, but I also didn't want to have to cut the plug welds that welded the inner fender skin to the outer fender skin, so I chose to do just a square. Anyways, what I did was measured from a known point on the fender (I chose the crease at the bottom) to the corners of my cut area, and transfered those measurements to the patch panel. I then made sure the area was roughly the same by holding up the cut out section, and once I was satisfied, I made my cuts. It's slightly terrifying cutting up a brand new section of patch panel, until you realize it only cost like $13. Then its not so bad.
Once that's cut out, fit it up to the gap in the fender, and assess the fit. I ended up having to cut a tiny sliver to fill about a 3/8" gap, but the rest of the panel fit surprisingly well.
I think the patch panel, while a very decent fit, wasn't quite perfect. The curvature of the panel didn't match exactly on top, so I had to hammer the panel in to close in the gap. This also created a bit of a bump between the patch panel and the fender later, but that got smoothed out with grinding and body filler.
Now, it's time for welding. It's a tacking fiesta. Tack weld one spot to hold it in place, and move on. You never want to heat up one area too much or else you could end up warping the panel. And if you blow a hole in the metal? DO NOT CHASE THAT HOLE. If you keep trying to fill it right then, you'll just chase that hole all over the fender. Wait for the spot to cool down, and then come back later and fill it. For reference, I had my MIG (Millermatic 251) set anywhere between 14-15 Volts, with the wire feed between 90-125. These settings change though depending on what welder it is. Once you do all the welding (It takes forever), its time for grinding and body work.
Grinding is just grinding. Dont grind too much away, you just want to get the panel smooth. Working with body filler is a little tricky. Once you have the filler and hardner mixed, you really do need to work fast. If there is a reasonably large sized bump to fill, do it in two steps; dont just try to pile a mound of bondo in the gap. Make smooth, quick applications with some sort of hard but flexible card, and once the bondo starts hardening, don't keep working it. Let it harden, sand it down, and apply again if needed.
As you can see, not all of my welds needed body filler, and I was quite happy with that. After sanding, I realized I didn't have sealant primer at the shop, so I ran home to spray some on. As I got home, it started drizzling, so I hastily sprayed the primer on, and it ran a bit (that explains those ripples in the paint). I'll sand that down today, and everything will be nice and smooth.
Overall I'm pretty pleased with how the panel came out. It's not perfect, but it sure is a whole lot better than before. One patch panel down, a lot more to go.