Back in the day and maybe even today, used motor sprayed to the underside would work. Had to be reapplied every fall. Today some of the modern outfits use a similar Oil type spray. Rust Kote is one.
Pete
Yep. Growing up in the rustbelt of Rochester, NY, we sprayed the hell out of cars & truck with used motor oil, mixed with Kerosene or diesel fuel.
Oddly enough, vehicles that leaked engine oil, transmission oil, or hydraulic oil resisted rusting much, much better. Eliminate the free path for oxygen and the rust stops.