Just as a matter of interest I can tell you of a product that I have been using for 20 years. It is called RustCheck. They have dedicated centres to apply the stuff but it is available for self application, usually used by construction companies or farmers with heavy machinery. The centres drill holes in many areas of the vehicle to make sure of total inside coverage. They use rubber plugs to seal up the holes. I first used it in Montreal ( an area of heavy salt application in the winter). I had a 1981 Chev Malibu that I bought new in 1982. A bit of a side note on that car: That particular model was one of about 10,000 manufactured for the Iraq government. They were to be gifts for returning soldiers of the Iran/Iraq war or for families of soldiers who had been killed. They had quite a few special features one of which was a 3 speed manual transmission,floor shift. Lots of other things but I could list that later. The Iraquis refused to accept the cars on some pretext so they were put up for sale in Canada. I kept that car until 1988 and it had no rust on it at all! The next vehicle was a 1988 Isuzu Trooper. (A GM affiliate so guess I can mention it). At this time I was located north of Toronto in a snowbelt area where they dumped copious quatities of salt in the winter. After 3 years, other Troopers that were not RustChecked were a rusty mess. My vehicle lasted me until 1997 when I bought a Chev Silverado 4x4. The Silverado still looks mint although this area I am in now is not quite so bad for laying down the salt. My 1946 Chev is due back from the painter anytime soon and it is going to RustCheck as soon as I get it. One of the Vintage Vehicle Associations endorses this product and I'll check further into that and post it.
Anyone interested could contact Rust Check Corporation, 6175 Danville Rd. Mississauga. Ont. L5T 2H7, Canada