Finishing your weld?

[80proZ]

I'll agree with most of what RobM said except the gap.You need a slight gap(about .023 mig wire) around the entire patch.
What will happen when you start to tack is the weld will cool and shrink,drawing the patch closer to one side of the hole and leaving a larger gap on the otherside.If you make the patch to tight,it will make the seams rise above the surrounding area,making it much harder to make dissappear.
TIG welding floor pans would be a waste of time IMHO.The seams and surrounding area would have to be spotless to make a good weld.Welding with a MIG is much more leinent and forgiving than the TIG.And grinding it down takes very little time.Personally i use a 4 1/2" grinder and cutoff wheel to do all my cutting and a 120 grit flapper wheel to grind down the welds.
I just finished some patches on my floors today,except for grinding, i'll post some before and after pics tomorrow.