I just recently experimented duplicating spot welds and the best so far has been to take the ball end of a hammer and with a wood backer, set the ball end over a 5/16"drilled hole where your plug welding ,before welding of course, and hit your ball hammer with another to create the dimpled depression into the panel surrounding the plug. This creates the indention in the surrounding metal of the two joined panels like you would get when spot welding metal under the pressure of the tongs. After plug welding, and grinding, use a die-grinder with a flat top carbide barrel cutter and with a steady hand burrow into the center of the plug weld just enough to create the effect. Putting a little angle in them makes them look even more reaalistic, then finish with a 1" wire wheel to clean up the edges and swirl marks. There may be a better way but so far they look great after you get the hang of it...just practice on scrap first as the die grinder will get away from you quickly...gouging the surrounding metal.
