Here's some image and pictures outlining the steps I took to repair rust on my 1990 4runner. It was getting so bad that when I drove on a gravel road or in the rain, the tires would throw up wet goo and it would get inside the rear panels. If there was enough of it, it would run out from behind the plastic interior panel and soak the carpet in the rear cargo area. Since the rest of the Toyota 4runner was pretty rough, the fix didn't have to be particularily pretty; it just had to work.
1. I removed the passenger side of the rear bumper. It is supposed to be bolted to the body in a few spots, but since the body was rusted so bad, I just had to unbolt the side section from the main bumper, and remove one bolt inside the rear fender well.
2. After getting the bumper off, I had to figure out what to do. The outer quarter panel skin is quite thinner than the inner piece of the body, so the outer was completely rusted but the inner piece was still solid. I decided to grind along the lower pinch seam to disconnect the outer skin from the still good inner skin.
3. The next step was to cut out the outer skin high enough that it would get me above the rust into good metal. There was some spot rust that I decided wasn't serious enough to completely cut out, so I just ground down to bare metal. For some reason I don't have a picture of it completed, but here is the work part way done.
4. When deciding how to attach a new piece of metal over the hole, I decided to bend the metal up along the dotted green line. This would provide a nice little holder for the bottom of my replacement piece of metal, that would not only hold it in place during the fix, but ensure I had a nice straight line along the bottom.
5. Cut a piece of replacement metal and weld it on along your newly bent lower seam, and aroudn the other edges. I had a chunk of galvanized sheet around, so I opted to use that. Just remember when welding galvanized, don't breathe the fumes as it is poisonous. I spot welded about every inch or two just to hold it in place. I welded a few spots, and then bent and welded as I went aroungd the sheet. The trickiest part is trying to bend the metal. Since I wasn't too concerned about looks, I have some wierd bends in the sheet and it doesn't look factory. Considering the rest of the truck, this is about the nicest part now anyways!
The real issue is when do you stop? The rust just kept going, so I opted to stop at the fender line and come back in a year or two when it rusts through my body filler. I tacked the rusted out outer skin to the stronger inner skin, painted it with rust converter and called it good. After all, it's an offroad vehicle, not a show truck!
6. Add your body filler and paint it!