Wire size is important when working on sheetmetal. I got a good deal on a crap load of .030 wire but it doesn't do me any good for the car because it's too big. .023 is about right. The last thing you want to do it blast a bunch of holes in the work and then spend most of your time trying to fix them. It can be like playing golf sometimes.
And don't believe it if somebody tells you you can use flux core instead of gas. They're lying.