For welding sheet metal or other thin steel solid MIG wire works really well. Like the OP said though it is very hard to control being so thin. The mild steel TIG rods that I have are ER70S2 where as the solid MIG wire I got is ER70S6, I think that designates a higher silicon content, but it does work on thin steel very well if you can hold on to the thin wire. And just like using the ER70S6 MIG wire for thin steel, aluminum MIG wire works great on thin aluminum as well.