Cars in northern states rust from the bottom up rather than from the paint inward. The reason is that road salt does its damage in places that stay wet and are never cleaned.
Salt keeps water liquid and conductive
Rust requires water and oxygen at a steel surface. Below freezing, water becomes ice and the reaction largely stops, which is why very cold dry climates preserve metal well.
Dissolved salt lowers the freezing point, so the brine on a winter road stays liquid at temperatures where plain water would not. The reaction continues through the season.
Salt also makes the water conductive, which speeds electrochemical corrosion by letting current flow between anodic and cathodic areas on the same panel.
Brine reaches places spray never does
Liquid de-icers applied before storms coat roads in a film that atomizes under tires and drifts into every cavity in the underbody.
Frame rails, rocker panels, subframe boxes and suspension pockets all have drain holes, and those holes clog with road debris over years of use.
A cavity holding salted water dries slowly because air does not circulate through it, so the corrosion continues long after the car is parked.
Dissimilar metals accelerate the process
Modern vehicles combine steel, aluminum and magnesium components fastened together, and where two dissimilar metals meet in the presence of an electrolyte, one corrodes preferentially.
Manufacturers separate them with coatings, isolators and specific fastener platings, and those protections degrade with stone chips and repair work.
This is why corrosion often appears first at brackets, fastener heads and repair seams rather than in the middle of a panel.
Brake and fuel lines fail before bodywork
Steel brake lines run the length of the vehicle in the airflow under the floor and are directly exposed to spray. They are also thin walled.
A line can appear intact while corroding from within its coating, and failure typically arrives during a hard stop when line pressure is highest.
Fuel and brake line replacement is a common major expense on older vehicles in salt states, and it often exceeds what the body condition would suggest.
What actually slows it down
Rinsing the underbody during winter thaws removes accumulated brine before it works into cavities, and is more useful than washing the paint.
Keeping drain holes clear matters as much, since a blocked rocker panel holds water indefinitely regardless of how often the car is washed.
Oil-based and wax-based undercoatings are applied annually in many salt regions, working by excluding moisture rather than by sealing the surface permanently.