Butt Welding 18 Gage w/MIG?

Your biggest hurdle is is heat. The 18g is light so turn down the mig and set it to stitch weld. The problem is that you want to hammer weld everything. And with a cold or sloppy weld it's hard to do. What gauage wire are you using? Check this out: http://www.airgas.com/content/details.aspx?id=7000000000143

Remember be patient and practice on the old scrap.

Hope that helps.