Hi all, I know this is really a novice question to ask, but I am trying to MIG weld in patch panels on both fenders (in the typical rust areas on the bottoms behind the front wheels). I was thinking of doing a butt weld, as the patch panel has a nice straight edge to work with, and so as to prevent any overlapping metal that will collect water later. What I was wondering is should I weld from the front or the back of the fender (or both)? From either side, if I were to then grind down the welds flush, would it be strong enough, as I believe the fender gets bolted in at this point. SHould I be concerned about the weld looking ugly on the inside fender? I always want to clean things up nicely, but not at the risk of weakening the weld I put down. I always appreciate the advice I get here. Thanks.
Mike

When u butt weld u should have enough penartration to have the weld go right thru.
If concerned about the inside join, hold a brass dolly behind, thu this should not be neccessry on a good weld and bit of practice...turn the heat up as high as possible (not blowing holes)and wire down so u are almost tig welding.
Stitch, hammer, stitch so as not to get too much heat into the panels and warp. This also allows to use a higher heat.
Take your time

Buut weldin will not cause any problems. Weld from either side, it really doesn't matter. Only do a small area at a time and move, a damp rag is useful to keep the metal cool. When you finish welding, grind the welds in the same fashion, a small area at a time and move and cool, this will keep distortion and cut downon work later. If welded properly the weld will fill the gap between the old and new panels, there should be no way water could get in. As for strength, the weld will be stronger than the steel, just don;t grind the welds too thin.

Thanks very much, I'll give it a try!

If u havnt done it before...practice on some scrap.
Also if u use a slightly heavier gauge panel steel to the body panel, with the outside flush, and starting the heat on the patch panel 1st, u will find it easyier

Good advise. Do a little, let it cool, move to another spot, weld, move to another spot, etc. This will eliminate warpage. Really can't add much else to what has already been said.

Something that works good for me is when I need to grind the welds down, I use a flapdisc instead of a regular disc in my angle grinder. They work slower so you don't thin the sheetmetal too much ....