Butt Welding 18 Gage w/MIG?

WOW, thats a big gap you are trying to fill with no backing plate. Even 1/8 is big, you need practically NO gap. If the the heat is making its way all the way through the panel as you say, then the heat is more than enough to melt the base metal and merge together.

To give you an idea what I am saying, in welding you draw the base material into the weld and just give it filler (which will be your wire). This is how you are supposed to set up your welds. Your base metal should be melting along with your wire to form good "penetration". When you are welding thin metal like that it will burn back when you hit it with the arc, so you want your panels to be right next to each other, so that when it burns back it wont open the gap so far that the wire that melts cant fill it.
I hope that helps more than confuses you. LOL Either way make it NO gap and itll be much easier.