9/11/2005: Metalwork 7: Welding a patch.

Another important psychological barrier crossed.. This time not only did I cut a nice hole in a very visible part of my car, but I welded a patch in! I was worried about whether or not I could do this properly. I’d done a lot of reading and some practicing, but still, it’s scary stuff! Finally I couldn’t put it off any longer.

I started by cutting out the bad section of this front fender. The section I cut out is sitting on it to the right. As you may be able to see, the reason that I could not repair this any other way is that someone drilled multiple holes and used a slide hammer on this area, right on the body line. Filling the holes and getting it all straight wasn’t really practical. So instead, I cut out the affected area so that I could graft in a piece from the other front clip.

Here i’m starting to cut out the patch from the other car. To make things interesting, since there was some rust pitting on the area I wanted to take a patch from, I actually took the patch from the opposite side of the car, then flipped it around and tweaked it a little to fit.

On both cars, I layed out a tape line a certain distance from the front of the fender. This reference line let me make sure that the patch was lined up appropriately.

I held the patch underneath the fender, then used a scribe to scratch a line right along the opening. I trimmed the patch right to that line carefully and fit it. I used a combination of a long-reach vise-grip welding clamp and a magnet on the underside to hold the patch in place. I think i would have had a better result if i’d had a few more of those long-reach clamps though. I couldn’t find anyone locally that sold them though, so i had to make do with the one I had on hand.

I’m welding using J.W. Harris Twenty Gauge 0.030” MIG wire. This stuff is supposedly a lot nicer to work with on thin panels than normal mig wire, despite being 0.030 instead of 0.023” thick. But since smarter people than I seem to like using the heavier wire, even on thin sheetmetal, who am I to argue with them?

I began by tacking every inch or so around the patch. After each tack, I ground it flat using the edge of a cut-off wheel, then used a slapper and dolly to work the welds as necessary to keep the patch flat and even.

Then I “stitch welded”, connecting the spot welds with inch-long weld beads. This is definitely the harder way to do a patch, but it gives a nice result. This is only practical when you have good access to the back of the patch though. After welding each inch, the metal will shrink somewhat, so you need to work the heat affected (blue) zone) on dolly to get it back into shape. If this isn’t practical, it’s probably safer to skip around doing only tack welds, overlapping them until the whole seam is filled. This minimizes heat as much as possible.

So again, grind down the weld with the edge of the cut-off wheel and work with slapper and dolly. A hardened dolly is necessary here, since the weld is relatively hard stuff.

And I continued around the whole patch, making sure to work each welded section thoroughly before I moved on. This takes a long time to do right.

The final result is pretty good, but I did make some mistakes. The initial fit of the patch really wasn’t quite perfect, so it’s got a bit of a low spot on the outboard side of the crease. I’ll try to fix that as much as I can with hammer and dolly, but if that fails it’ll be fixed with filler or lead later. It’s close though, a lot closer than i could have gotten without doing this patch.

I’ll post a photo once I finish tweaking this and spray it with primer, we’ll see just how invisible it ends up.

As far as this special MIG wire goes… It seemed to work well, but since I lack a good reference for comparison, I can’t really give a wholehearted recommendation for it. But it worked fine, and other people like it a lot, so i’ll continue using it in the future.