Unless the panels on the vehicle are completely disassembled, ie; skins removed from skeletal frames, there is no way to protect areas with epoxy, etch, primer or any type of paint. However, most vehicles, if not all, built during the 60's had the metal conditioned for rust prevention. It might not have been the best rust prevention method but, it was the choice used from the manufactures back then. I can't speak for pre 1960's, don't know anything about the process used back then. With that said...................
The best one can do in my opinion when "not" disassembling panels by spot weld removal, is apply epoxy either spraying, brushing, rolling or even pouring it on and letting it absorb it's way into cracks and creases. It's my understanding that lite surface rust can be slowed down or halted from continuing to spread once covered in epoxy.
Here's another way to look at it. You can have a vehicle dipped to remove paint and rust, that process is going to remove a percentage of rust, if not all of it, that has formed between spot welded panels. How to protect those areas without disassembling them? As already stated........spray, brush, roll or pour epoxy over those area's.
From my point of view, I would never dip a vehicle that I wasn't willing to disassemble panels via spot weld removal so those areas could be properly rust protected. The manufactures rust protection is better than nothing. My 50 plus year old car is proof of that. I have some area I can see have discoloration, that is the starting point of which rust will continue to form. I have simply cleaned it up the best I can and coated it with epoxy. Those areas will last another 50 plus years if not longer. I can say without hesitation my car is rust free.