While most of the vintage tin around will have left the factory some 60 odd years ago with absolutely nothing between the panels, something has got to be better than nothing at all. But for longevity, which would you think will hold up better, a non catalyzed finish out of a spray can or a catalyzed epoxy finish? Which do you think will offer better adhesion to the bare steel panel? I've got some panels here that I've done hammer and dolly work after the SPI epoxied finish was applied, with no detriment to the paint adhesion.
As far as the present day manufacturer's recommendation for using the weld through in repairs, most body repair facilities will not take the time to spray a good epoxy finish between a lapped metal weld seam as they would need to mix up paint, clean a paint gun, and allow for cure time. You'd have better luck getting anything actually used at all if it was EASY and FAST. Spray cans are easy, and if you do some research into both this and I-CAR recommendations for other repairs, you find that much of the guidance given is less about being the most optimal method of repair and more of what is easy or easily duplicated so that they would have better luck in getting something done in the course of a body shop effecting that repair.
Darcy brings up a good point,
One sure way to avoid this is to not lap weld
Many of the lap joints used at the factory are not because it is the
best method, but because it is a
faster method of assembly. Faster down the assembly line, faster out the door, faster to rust so you'll need to buy a new one. Lap joints, Pinch welds, or whatever else you'd like to call them are the main source for rust issues in all vintage vehicles. This is why many of those seams have been removed from the 55 Wagon we're presently working in the shop, in favor of a butt welded seam that can be finished on both sides, and epoxy primed on both sides to seal out the elements in hopes this would be the last such repair..
