Tonight I welded in the small rear fender patch pieces I fabricated previously. Since the patch steel I bought was galvanized, and welding-up galvanized steel can emit some dangerous fumes, I first used a wire wheel to remove much of the zinc coating from the patches, particularly around the edges. I also was sure to weld outside with good ventilation. The patches fit well within the cut-outs, which I cut to size. For the thin sheetmetal of the roadster body my MIG welding set-up consists of .023″ solid-core wire with CO2/Argon shielding gas.
So with the patch in place, the first thing I did was tack weld the patch at the top. Again because the sheetmetal is thin, rather than try to lay down a continuous bead around the entire patch my technique is to weld the patch with numerous spot welds, working around the patch until the entire thing is welded in. If I used a continuous bead I would probably heat the metal and burn through before long; using multiple spot welds enabled me to allow one end of the patch to cool a bit while I work on the opposite edge.
As I worked my way around I also shaped the patch a bit to try to conform it to the shape of the fender lip.
The result was pretty ugly, but strong with good weld penetration without blowing lots of holes in the fender. The welds will clean up nicely later. I also welded in the two patches on the left rear fender.