The best I have personally been able to do is.... coat the entire weld area with SPI black epoxy, allow to cure 24 hours or preferably more, mock up and align the panels in prep for welding, then use small carbide bit (dremel, die grinder) through the plug weld holes to grind the epoxy back. Fully plug weld the panel in place. Then wipe down with wax and grease remover, allow the cleaner to flash off, and using a brush, saturate the entire weld seam in SPI black epoxy again. Epoxy literally running right through the seam and dripping out the other side is desireable.
It's not as easy as a rattle can with weld through primer, but hopefully it will hold back corrosion a little longer.
After it's topcoated, use cavity wax where applicable.
Overkill? Maybe, but I'm from Detroit area, we need all the help we can get.