Gas for sure, argon/co2, 75/25.

My Miller 252 with .035 wire does not like to do the low range butt welds on sheet metal, but lap welds work out OK. I use the same wire I weld everything else with.

For weld clean up, I use a 3/16" cutoff wheel on a die grinder to take the high spots off. Since I mostly lap weld, I also flange the sheet metal and leave the step down a little and weld in the low section between the step and the new metal. A lot less grinding. I don't hang many quarters, so the lap joint is not an issue. Gene