Rustproofing a car that has been exposed to the elements for years is not a good idea. Things rust from moisture trapped inside the metal and applying some sort of rustproof coating over metal that has moisture in it only accelarates the rusting process. Now if you were putting on a new hood or a new fender that would be fine. If you have rust spots under your hood you may want to apply a product like "Rust Mort" which stops rust from progressing.