Butt Welding 18 Gage w/MIG?

Low amps and a decent amount of gas, will make for nice Tacks. You will have to grind if you are looking for a nice flat joint though. Its nearly inevitable that you will have some pin holes just because you are tacking and trying not to get the metal hot. My suggestion is that you tack it carefully all the way around. go back and grind it all, then take a light to the back side of the panel to see pin holes. Thats if you are trying to be that picky. At that point be sure to at least wire brush any residue from the area of your pin holes, and tack them again. Im sure you know this but be sure to move around the panel alot with your tack placement.