I have a couple of MIG welders and usually use .023 wire; I think it is ER70. The smaller diameter wire both limits the amount of heat and makes a smaller bead. My first MIG, bought about 25 years ago, was really made for heavier work. A few years ago, I bought a 110 volt Lincoln and it both made welding easier and did a nicer job on thinner metal. Now I don't use the old Systematics machine on anything less than 1/8" thick.
If I am doing exterior sheet metal, like a fender bottom, I tack the patch in place and then weld short beads, about 1/2" long, skipping around the patch to keep the heat down.
I usually use flap-wheel disks to grind the welds and they don't generate a lot of heat. I also learned my stud welder works pretty well for shrinking small areas if that is needed.