Rust is formed by a combination of water, air and steel. It is an electrochemical corrosion process in which the water and air break down the composition of the steel. Rust can form on the metal parts of a car, particularly in snowy climates where salt is used to deice roads in winter. Salt also contributes to corrosion.

Rust can begin to spread in areas of the car exterior where paint has been chipped away, exposing the steel underneath, but it can also start from inside the car body and spread out. Rust-proofing is intended to spray rust-inhibiting chemicals or sealants to inside sections of the car body, such as the inside of door panels, quarter panels or fenders.
     
Rustproofing is different from rust prevention undercoating, which involves spraying only the undercarriage of the car. With rustproofing, specialists drill small holes in the doors and insert a spray nozzle tool that applies the sealant. Then the hole is plugged up.
     
There is considerable debate about the need for rustproofing. But, for people who do live in snowy climates and plan to keep their cars for more than three or four years, some do advise getting rust-proofing to help the cars look better longer.