Any mig welder flux core or gas shielded even on a very low setting will blow through on a continious weld. Trick is to make several spot welds then move down the line and make more spots and let it cool in between then more spots and repeat till you get it all filled in. One thing that will help you is a piece of scrap copper clamped in on the back side,it will do a lot to help you from blowing through and the weld will not stick to it. Harbor freight sells a copper paddle with a handle just for this purpose for around 15 bucks. I prefer oxy/acetelyne with a very small tip or a tig welder for sheet metal. Oxy/acetelyne bead is the easiest to dress out and planish but causes the most warpage. I prefer the tig weld as I feel that I have more control of the weld and can just sit on the puddle without blowing through for good penetration. Never used a flux core mig but my experiance with mig welds on sheet metal is a very hard weld thats tuff to grind and very brittle when trying to planish. All welding on sheet metal is gonna cause warpage with any of the processes.