Why did I do this?
Our '96 Buick LeSabre has had rust inside of the passenger-side rear wheel-well since we bought it a couple years ago. Apparently this is a somewhat common problem on these cars, and now I know to look for it before buying one. I decided that this repair had to be done now for two reasons. First of all, whenever we drove in the rain, water was getting into the trunk from the tire spray. That's not good. Secondly, it was getting worse. But why do it myself? I decided on doing it myself because I could do it cheaply, and because it was in a place where it would probably never be seen, so an ugly repair would be just fine.
The repair process, in great detail
I really dug into this project with only a vague idea of what I was getting into, since the rust was still mostly covered loosely by peeling undercoating.
The first step then, was to scrape off any undercoating that wasn't firmly stuck in place, using a screwdriver. This revealed three holes, two in the rear, one in the front. Then I scraped and wire-brushed off as much loose rust, paint, and undercoating as possible, followed by a good blast of air from my blow-gun to remove as much dust as possible. I even wiped it down with a damp rag (carefully - the rust was sharp in places). Then I brushed on a thin coat of rust converter.
Converted rust
More converted rust
From inside the trunk
Rust converter is a great thing! It converts rust into a hard, black, paintable, primer-like substance. The product I used was about the consistency of a good thick latex paint, and it spread farther than I thought it would (I had to use up the extra on other spots under the car). I'd never used it before, but it seems to have worked great.
Twenty-four hours later, the rust converter had cured, and I was ready to move on. The original plan had called for one large sheet-metal patch, but the shape was more complex than I had figured on. Two smaller patches would be easier to bend into shape. I just happened to have an old Grand Prix fender sitting in the garage (thanks Drewski!), and it was just large enough to cut patches from. First I made some paper templates, holding the paper up in the wheel-well, and tearing off anything unnecessary. Then I transfered the shape of the paper onto the fender with a marker. Finally, I cut the patches out with a hacksaw. If I were to do this again, I think I would have spent some money on some air tool to cut sheet metal, or at least a hand shear. Cutting sheet metal with a hacksaw just sucks.
The patches...
Bending the patches into shape wasn't hard. I used a hammer, vise-grips, and my hands. First, I got them close to where they had to be, then I beat on them a bit while I held them in place in the wheel-well, to "fine-tune" the shape. When I was satisfied, I moved on to the next step.
I briefly considered just sticking them in place with an adhesive sealant, but quickly decided that I didn't trust that method. I went with sheet metal screws instead, so drilling pilot holes in the patches and car was next. Holding the patch in place, I drilled one hole, and temporarily installed it's screw. Then I did the same thing somewhere on the opposite side of the patch. With those two screws in place, I could drill the other holes without anything shifting around too much, and causing alignment issues later. As you can see in some of the pictures, there are areas of the wheel-well that are double-thick, or reinforced. After breaking off one screw while trying to tighten it in the thick area, I decided to go up a size for my pilot bit in the thick stuff, and then the screws went in fine. You'll see I did think, and go with a hex-head screw, so I could put some real torque on them if necessary.
Patched and caulked!
Sticky and white!
Once all of the holes were drilled, I removed the panels again, and drilled the holes in them larger, so that the threads on the screws wouldn't catch on them. It's just easier to work with them that way. Then I de-burred all of the holes, roughed-up the paint with sandpaper, wiped them down with some alcohol (not sure if there might still be wax on the old Grand Prix paint), and gave them a quick coat of auto primer.
When that was dry (about 15 minutes), it was time to actually attach things permanently. I bought some GE Siding and Trim caulk, which was a silicone/polymer blend of some sort. It claims to never shrink, be permanently flexible, and be paintable after one hour. I gooped on a liberal coat of the stuff along the edges of the patch, especially where it overlapped with the body seam, which indents just a bit. I also gooped some caulk on that seam on the body itself, just for good measure. Then I screwed the patches in place. The caulk squished out from most of the edges. I caulked any edges that were still dry, and also some other questionable areas of the wheel-well that hadn't been covered by the patches. I smeared it around with my finger, and made it as smooth as possible. It's an ugly repair, but I might as well make it as good as I can.
A couple of hours went by, and I went out to check on things. The caulk was still pretty tacky. Another couple of hours later, it was still tacky. Finally, another four or so hours later, I just moved on to the next step anyway, despite it still being rather soft and somewhat sticky.
Finished! (but still wet)
Still wet (but finished!)
The next step was to spray on some undercoating. I had never used undercoating, so I was somewhat unprepared for the very strong vapors that it produces. It's like spray-paint x5. In hindsight, this was an outdoor project, not a garage project, even with the doors open and a slight breeze. I made it work though, mostly by running in and out of the garage, and holding my breath as necessary. Undercoating also needs to be applied in thin coats, I found. Thick coats get bubbly and mushy. I actually was able to squish it in spots with my finger, and have it seep clear fluid. Once it was "purged" in that way, it dried fine, but it took longer than it should have. The first coat shrunk as it dried, and left cracks over the white caulk. The next day, I applied a second coat, and it looks mostly good now. I may yet add a third coat in the spring, but this will do for now.
End results and thoughts
I'm quite happy with how things turned out. In hindsight, I might have gone with a different caulk/sealer. A week later, I can feel a spot of it inside the trunk yet, and it's still slightly mushy. I suppose that is just how the stuff is. It seems to be holding up fine, but time will tell if it is too soft. I'm very glad to have this done. The front hole, I think, was actually leaking to the inside of the car and not the trunk. I won't cringe anymore when we drive it on wet roads.
This was a pretty cheap project. The rust converter and undercoating cost around $10 together, and the screws and caulk also cost around $10. If I had to buy metal, that would have been another $10-$20, but I had that. I never did get a quote from a body shop to get this done, but I'll bet it would have cost me more than $20!