As far as the gap ... I usually leave about 1/32" (.031) all the way around ... that way the weld can penetrate BOTH peices of metal and you can then grind it flush.
As far as the grinding. Don't use your 4" side grinder witha 36 grit stone on it for your finish grinding. Use it to knock down the "high spots" and then switch to something like a 50 grit PAPER/RESIN type disk that goes in an air tool. Use it sparingly .. .then switch to something along the lines of an 80 grit. Work your way up to around 120 grit and you shouldnt have too much problem with "gouges" left by the grinder. It also helps to cut down on the heat. Remember .... HEAT IS BAD ... it causes warpage. Less gouges and less warpage equals less filler.
P.S. - Don't forget the beer ... HEAT IS BAD & the beer helps to keep YOU cool!
Here is a pic of mine after paint ... sorry, no before pics. I didn't smooth the spot welds or the driver's side ... although I wish I had now.
