I decided to butt weld most of the things I have done so far. I use a piece of copper roof flashing folded over double as a backing and clamp it up very snug to the pieces being welded. In some cases when my weld pool is nice and flowing it fills the gap very nicely and the back side of the weld looks very very smooth. I use a MIG with some very fine wire. I think it may be .022. With the copper it helps with the heat too. I still get in too big of a hurry but it does help. I don't know as much as the others here but that has worked well for me.
