If you live where road salt is used, wash your car’s underbody in mid-winter with a water hose and spray gun. With a stiff wire, unplug drain holes in door and hatch bottoms.
At the end of winter, repeat and also spray the engine compartment be-hind the headlamps. Underneath the car body, flush out road dirt by spraying between the bumper and its rein-fore ing bar and be tween the fender reinforcement and the fender itself (if there is a gap). Also spray underbody mounting brackets.
Work a stiff wire into holes in the underbody box sections to loosen packed-in road dirt, then flush it out with the spray. Cut away cracked undercoating, which can trap water and promote rusting.
Dry and recoat the area with an aerosol undercoating.