A friend of mine has a 1964 Ford which has been all over the world since he bought it new, includig salty areas. No rust, ever. When he got it, he took it to the carwash and thoroughlky cleaned the underside, then sprayed all the out-of-sight areas with plain raw (not boiled!) linseed oil. When he does any work underneath--e.g., muffler hangers, etc.--he touches up the surounding area with a brush and more linseed oil. ( The raw oil stays tacky and malleable; boiled oil makes a hard surface which will crack and hld in moisture.) This will work over rusted areas as well, and will not seize bolts and nuts like paint will; not harmful to wiring or rubber parts, either. Just make sure everything is pretty clean and completely dry before you spray. For the skeptical: take a whiff of raw linseed oil, then take a whiff of the various expensive rustproofing compounds. Somethng smell familiar?

Lots of luck!