WW2 paints might very well have had lead in the product, which would assist in protecting the substrate. Remember hearing of primer called 'Red Lead'? It was all the rage in 30s to early 60s, and the name speaks for itself. I'm not sure if that used on military vehicles, but in those days they encouraged the men to smoke too, so workplace health & safety not such a great concern as it is now.
I always use single pack primers, and always start with a light but complete coat of etch primer, for excellent adhesion and protection. Etch is only good on bare metal, so I should say that I always have things sandblasted to begin with. Nothing can hide from the sandblasters gun. Bare metal MUST be sealed as soon as possible, and neither etch nor primer/filler is protected from rain or condensation, so also MUST be kept out of the weather. Be careful using etch or any type. It can contain harmful fumes that damage the lungs. In fact, best avoiding ALL paint fumes, so be sure to wear a mask atall times. I finish coat with Protec Barrier enamel with hardener additive chucked in for good measure. Not actually 2-pack, but does have some of the same properties. My primers on the other hand, are always acrylic. You can paint enamel or Poly paints over acrylic, but not the other way around......ever. If body filler is used, as it invariably will be somewhere, prime once body filler is sanded to completion, then apply several coats of spray putty. Once again, I use acrylics there. If using full-on 2-pack, a combined undercoat/spray putty can be used (2-pack) but in long term restoration process, I would advise against it. If left for weeks or longer, it can get extremely hard, and will then be like sanding granite! Some are worse than others, but I can't say which are the hard ones, so you roll the dice there. In the good ole days we deliberately induced that hardness, by leaving a resto vehicle under primer for a number of weeks, and then did a thorough dry sand with glass paper on a long body file. You sure worked up a sweat, but got rifle straight panels (if sanded correctly). Then the priming would be done again, and painted to completion about a week later. Ah, the good ole days!
Now, how weather-proof are flat paints across the various types........., that's a whole other can of worms!
