I agree with knowledgepower. Some forms of rustproofing involving the drilling of holes could create additional issues. And undercoating is as honest a business as the diluted spray-paint 'sealcoating" of driveways.
If there is a part of a model's design that allows water (and salt) to sit and act on the metal, you could spot-treat that specific area with a spray can of rubberized undercoating sold in autoparts stores for a couple of bucks. But nearly all cars made today have improved designs that don't encourage rust.
A high pressure car wash that cleans the underside is your friend in the winter.