Trust me when I say DONT use a rubberized undercoating. It can flake off, separate and water can creep underneath. A good, non-drying rust proofing product will seal out moisture like no other, and if you are unable to clean off all surface rust, a liberal coat will at least stop rust from progressing. Rust is almost like a living, breathing thing...it likes water, food (metal), and oxegen. Take any one of those away, and it stop growing.
I live in one of the worst regions in the salt belt. Unprotected cars can show the beginning of serious rust signs in as little as 2-3yrs, so if you don't do it right....you'll pay for it. There's a reason why 10+yr old cars are a very rare sight here.