[competentone]

The problem with "rust proofing" -- I assume you're talking about adding the tar-like undercoating -- occurs when the coating starts to fail. Moisture can get under the material and remain longer than if there was no undercoating, causing more damage.

Getting a good coat -- in an attempt to prevent failures -- can be especially difficult if you're applying the material to a 20-year-old car -- you might end up sealing in salts and dirt which will help cause the material to fail and work with the moisture bound to enter the failure to attack the paint/galavinized coating and finally the steel.

Personally, I'd leave an older car as it is and clean the underside regularly, dealing with individual chips/scrapes with touch-up paint, and avoid any undercoatings.