Number one rule: Clean Clean CLEAN! Always wirewheel (or whatever you can reach it with) clean your metal before welding. I use as hot a setting as possible without burning holes so that the bead will be as flat as possible.
Butt welding sheet can be tricky. For 18ga and thinner I use 0.024" wire. The thinner wire seems to be more sensitive to wire speed. Too slow and it will ball up on the wire - too fast and it will pile up like bird turds on your work. I like to go along and tack the entire edge straight so I end up with tacs about every inch or so. During this process I am lining up the edges as I go by using a jewelers (tiny) flathead and/or hammer/dolly. Then I connect between the tacs with a pulsed pattern zap-move-zap-move... with each tac right next to the previous and only about 0.5 sec wait time between them. Even then I get some not-so-pretty results on sheet sometimes. Tipping away from the weld (dragging) seems to help.
Next I grind with a standard grinding disc on the 4.5" angle grinder down to a few thou above the surface. Then I hit it with a flap disc to take it down flat with the surface. I'll go back and zap any pits as needed and finish them the same way. My last step is to hit it with the DA. I use anything from 60-120, heavier being preferred, but generally whatever happens to be on the DA
