Wash your car (undercarriage included) weekly during high-salt use.
Wax semi-annually. Apply a carnauba-based wax. Apply by hand in a swirl
pattern, let dry, then buff clean. If using an electric applicator, be careful. Too
fast with too much downward pressure will burn through the paint.
If you are going to keep your car for a few years and you live in a salt belt,
have professional rust proofing done on the car.
During the salt-use season, do not park your car in a heated garage because
heat triggers salt corrosion.
Inspect for rust annually and repair as necessary; don’t leave bare metal to
fester into a rust spot and grow.
When adding new parts to your vehicle such as a fender or door, make sure
they’re rust proofed before installing.