One trick I've learned to get good plug welds, especially when the holes are small, is to make sure that the arc from your wire starts in the center of the hole and then work outward. If your wire is too close to the edge of the hole when you pull the trigger the arc will be drawn to the metal at the edge of the hole and the sheet behind will not get hot enough to fuse properly. Also, I've used the weld-through primer and found that it doesn't really weld very well. It pops and smokes like welding galvanized. I still use it on the surfaces that can't be painted but I remove it from the area that will be welded.
