Hi Nova,

I am sure by now that you have discovered that welding sheet metal is a whole lot different than structural steel. Basically, instead of running a solid bead you have to do stitch welding every few inches. After you get to the end you will have to go back and stitch weld between those every so many inches. Keep the welds going between the stitches until it pretty much fills up between them. Of course, when you say MIG welding, I am assuming that you mean with the use of gas. The flux core wire sucks. There is only one good place that I can think of for using flux core wire and that is to hold the trash can liner in the bottom of the can.
I always weld with .023 wire. .030 wire takes too much heat and burns through too easily. With .023 you can always turn the wire feed up to compensate if necessary whereas using .030 you can't and it takes more heat which will warp the metal worse and get a nastier looking weld because of trying to keep the heat low enough to keep from burning through. In certain instances I have found that I can weld a steady bead for maybe an inch (sometimes more) and let it cool while working farther on down the seam. Problem is, longer beads= more heat. More heat= more warpage.
It always helps if you have room to work the weld to take the stress out of it. In my younger days I always used to braze the sheet metal but the flux sometimes caused problems later on while trying to paint if you didn't get I all off. You will see most body shops use a MIG welder. I haven't seen anyone actually use Oxyaccetelyne in body work with mild steel since I was a kid but I see it is recommended for doing automotive sheet metal. Mostly because the MIG welds with about 10,000 PSI tensile strength and the gas welding is much softer and more workable.
Another thing I am sure that you have noticed, when butt-welding steel of any size at all you will typically leave a gap between the two base metals the diameter of the filler rod. You can't do that with MIG because it will blow away. You will have holes everywhere. You probably knew all this anyway.