There’s a right way and several wrong ways to patch a rusted-out fender, some of which involve flattened-out Castle Lager cans. (You know who you are.) But the only really appropriate way is to butt-weld a neatly trimmed patch into the hole in the sheet steel. A lap weld will leave a lip that has to be disguised with plastic filler, and a crevice to collect moisture and then rust. We use these welding clamps to position the patches while we make a couple of tack welds, then remove the clamps to make a neat seam weld. The gap is exactly the right width to ensure full penetration.