I also use a Miller 210. I use .023 wire for sheet metal and .030 for heavier items. Gas for everything. Adjust your gas flow and play around with the heat and wire speeds. The chart on the machine isn't alway nuts on. For plug welds I usually adjust the machine up to the next thicker gauge of metal, so if it's really 20g I'll go 18g etc. I little hotter.
Make sure the area is ground bare and clean it if you need nice welds.
If the gap is too big it is easy to burn through. Like others have said take your time to "fit" the metal and gaps.
Wes
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