[MP&C]

I prefer to use epoxy primer instead of weld through. IMO a catalyzed primer is going to offer better long term rust protection than anything out of a spray can. As Eric stated, I also clean the paint out of the plug weld hole using a flattened drill bit. After spot/plug welds are complete flood the area with epoxy to seal. Here's a sample of how I did a seam on a lift gate, using HOK epoxy primer..

In order to use the spot welder along the bottom seam, we needed to clean off some epoxy primer where the spot welds would be located. Used and even spacing of 2" and marked both flanges...








In order to remove as little epoxy primer as possible, we decided to use the modified plug weld drill bit (the flattened one) over all the errant marks a roloc sander would make. In order that the flat drill bit didn't walk all over the place, another specialized tool was made....














Here's another close up of the modified bit...





.....and the "prepped" flanges...








Ends tacked in place, and spot welded the bottom flanges together...