I would agree with alschnertz that you'll be better off leaving that felt able to breath and dry out after getting wet, rather than covering it up with something that will trap water. If I were going to treat it with anything, I would say that letting it soak up silicone spray might be a better way to go than oil, but what do I know?

As for treating the bottom: I believe that at least the early PVs came out of the factory with just paint on their bottoms, and not really a great deal of that. My "62 certainly looked as if did, and there wasn't much of that paint left. The most effective way that I know of for fighting and/or preventing rust there is POR15, ZeroRust, or coal-tar epoxy. ZeroRust is the easiest to apply and probably just as effective as POR15 and a lot cheaper. The best way of getting the best adhesion is over lightly rusted or well abraded bare metal that has been treated with their brand-specific magic potions - again, ZeroRust is the most lenient in that respect. You can probably get away with applying the stuff over primer if the primer is well adhered. Short of removing it, I would just go after it with a steel brush.

I would not put undercoating over primer, especially not primer that shows rusted spots, unless I'd first coated the primer with something impermeable. Rust under under-coating eats metal faster than if left exposed to the air.

Mind you, these are just opinions based on my fairly limited experience with a couple of cars. So, no guarantees. ;-)

Bob S.
--
"62 PV544 (B20, M41), "71 142E, "93 240 Classic Wagon.