Most use angle grinder, hold at 45 degree to weld and lightly grind the weld down, little at time, you don't want to put heat into it. Also wear safety goggles and mask, you don't want to be breathing it. After you get that down, use a die grinder with 1-1/2 in. dia. sanding buttons to finish off to metal surface. You will accidently nick into surrounding surfaces but you will be filling with body filler to take care of seam anyway and smooth out. One thing you don't want is the surface to end up sticking out beyond the normal body line, as it will show up. Better slightly low so it can be filled with mud.
Another thing you will want to do is get a couple scrap pcs of the same gage material and practice welding on them to get your heat and wire speed correct. You can also weld a seam and bend it to check for strength. Shouldn't break where weld is or your too cold. You can also cut across that weld and check for penetration of the weld.
As far as welding on the front side or back side if you butting the pcs. together it shouldn't matter, as you should be penetrating all the way thru.
Practice makes perfect!
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