Butt Welding 18 Gage w/MIG?

Lots of good info here from alot of guys that sound like they know welding better than me but I just thought I'd throw in that I, like hydro used a copper plate on the back side and it helped greatly. In most cases I had to have help holding it but that was the greatest obstacle I encountered. You have a tremendous gap to fill so you need something to back it up IMHO.

I found a piece of 1/2" copper water pipe about a foot long smashed flat the last 2-3" of the end and bent at approx a 45 degree angle worked good for most areas. My buddy was able to hold it in areas with no trouble.