Rusty floor repair

[Ian]

I recently had to repair some rust holes in the floor, and tried out a couple of tricks that made the job a lot easier. I thought I'd post these methods, in case it might help anyone else in our community repair a common problem.

Our patient is a 1991 318is named Skeeter, which while not quite a 2002, does at least share some genetic material in the M42 engine, and these repair techniques are equally applicable to the '02. We discovered this lovely rust lurking under a heatshield by the passenger-side transmission mount.

The first thing I did was go to Harbor Freight and buy a couple of cheap welding blankets, and used them to drape Skeeter for surgery, so the cutting and welding sparks wouldn't hurt the interior.

There is some debate on how to repair holes in the floor. I've tried the overlap method, where you make a flange in the floor, and drop a patch into the flanged area, and then weld up the seam. The problem I had is that you have to weld the seam on both sides of the overlap, otherwise you trap moisture. The alternative is to butt-weld your patch. This has only a single seam, but requires time-consuming fit-up to get a patch that fits exactly. Otherwise, if you are not careful you end up blowing holes through your floor trying to fill gaps.

This time, I tried cutting the patch and the floor simultaneously to get a good fit. I first cut out the rust, and marked the lines I wanted for a patch with a big sharpie on the clean metal. Then I made a paper template of an over-sized patch, making sure that my sharpie lines extended beyond the patch template.

In this case the area of the floor we repaired had two curves to reproduce, so I used the high-tech shaping device in the next pic to reproduce the curves.

Then I tacked the larger patch over the smaller hole, and transferred the cut lines to the patch, using the existing sharpie lines on the floor guides.

Next I marked my cuts with a scribe, using the thick sharpie lines as a poor man's version of layout ink. I made the cuts with a combination of a pneumatic cut-off wheel with a 1/16" disc, or a body saw. In each case, I held the tool at a 45 degree outboard cant, ensuring that the two pieces would fit, even after accounting for loss of material due to blade thickness. When I was done I had a patch that fit almost exactly in the hole, saving me hours of time that I normally spend trying to get my patch to fit.

I tacked the patch in with my MIG welder, and then tried a second tip I had heard. For TIG or oxyacetylene welding you'd like a tight fit, but for MIG welding it helps to have a slight gap between pieces to get good penetration. Before I welded the patch in, I went back around the perimeter with my body saw (leaving the tacks in place), making a thin gap all around the patch. This gave me a nice consistent substrate for welding, and I did the usual skipping around the perimeter with my MIG welder, putting each tack into the root of the previous tack.

About half the welds were made from the passenger cabin, and half from underneath the car lying on my creeper. The welds are not necessarily pretty,but I got good penetration. Here is the patch from below to compare to our first pic.

In this case I discovered a second batch of rust towards the front after I cut out the first patch, so had to make a second semi-circular patch for the front. And then I discovered a third batch of rust towards the rear, discovered as I blew the paper-thin remaining metal away with my welder. So I got to practice my technique for a third time before I could reconstruct the tranny mount.

Finally, I treated all the weld and exposed metal with Picklex20 rust converter, and scrubbed all the welds with a stainless brush, and then covered the welds with POR15 seam sealer. This is great stuff, basically a thick version of POR15 paint. I didn't bother brushing it on, I just squeezed a bead out of the tube along the weld, and then wearing a pair of latex gloves I used finger-painting technique to work the sealer into the weld area. Then I coated it with POR15, and later I'll shoot some undercoating over the area to ensure it stays watertight.

I won't pretend to be an expert on sheet metal repair, but thought these ideas were worth posting, as they made the job much easier. I welded in an entire front floor in my M2 a couple of years ago using the flange method, and it was a lot of work. This butt-welding method is so much easier to do, and so much more waterproof, that I wish I had known about it for my first floor project.

[Bavarian Deuce]

Nice metal shaping and some good beads here and there. Very well said about overlapping and panel patching, it becomes a nasty moisture trap! Even when you use seam sealer later as the metal is not really coated/protected. Best way is to tack the panel and then stitch weld around in order to avoid overheating and warping your new panel and the likelyhood of blowing through. Kuddos for the belly side welds, laying on your back and receiving all those sparks is not fun.