I agree use the recommended sized wire for whatever gauge steel that the fender is (I'm guessing around 18 ga). Practice, practice, practice. I'm a certified tig and mig welder and lots of very small tacks is the right way to do it. In a butt weld that size you may have a few hundred tacks with no bead. Butt welding old sheet metal is like painting in that your prep, and post work is much more time than the actual welding. Just a heads up, when you apply a weld the metal tends to be pulled up or shrink in that area. As you weld try to think about spreading out this controlled warpage to make your hammer and dolly (invest in a good set) work easier later. Also to clean up the area a good body file is the way to go. It takes forever but a grinder puts a lot of heat into the metal and can actually warp a panel worse than your weld (if it's a careful weld).

Good luck and kudos to keeping that baby rust free
Dan Ferrier

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'78 300D 85,000 original miles drive her daily