Your car's underbody consists of the wheel arches and lower body frame. The area is prone to caked-on debris that is susceptible to rust if you don't remove the dust, dirt and grime. Rust can also form on your car's underbody due to moisture breaking down the metal and causing it to oxidise. You can stop the oxidation and rusting process by cleaning your car's underbody on a regular basis, using a cleaning solution that breaks down the debris and removes it.
Mix in a bucket 1 cup of car wash containing a degreaser and 1 gallon of water. Wear latex gloves to keep your hands clean and dry. Stir the soap mixture with a scrub brush or wash mitts. Park your car on a level surface -- flat driveway, concrete patio or other level surface.
Set your car's emergency brake. Loosen, but do not remove, the lug nuts on the rear wheels, using a tire iron or wrench. Use a floor jack to lift the rear of the vehicle. Place jack stands under each side of your car then slowly lower the jack until the rear of the car is fully supported. This allows you better access car's underbody. Remove your car's back wheels to access the wheel arches better.
Spray your car's underbody generously with water to break up any loose debris on the raised ends of your car. Apply the soap mixture to the underbody including wheel arches and the backside of any bumpers on your car.
Let the mixture stand on the underbody for 10 to 15 minutes to penetrate the debris. Scrub the debris vigorously with a brush or wash mitts to break up the debris and remove it from your car then spray the underbody with water.
Mix 1 cup of a foaming car wash concentrate and 1 gallon of water in the bucket and stir the mixture with the brush or mitt. Apply the mixture to the underbody to break up any stubborn or caked-on debris the soap mixture did not remove.
Let the foaming car wash mixture stand for 30 minutes to penetrate any stubborn debris then scrub the underbody with a brush or wash mitts. Spray your car's underbody thoroughly with water.
Dry any painted surfaces on your car's underbody with a microfiber cloth to keep any moisture from penetrating the paint.
Place your car's wheels back on its rear end, secure the wheels lug nuts and jack up the vehicle to remove the jack stands. Lower your car's rear end slowly to the ground. Tighten the lug nuts completely.
Loosen, but do not remove, the lug nuts on the front wheels. Lift the front of the vehicle and support it on jack stands. Remove the front wheels to access the wheel arches better. Repeat Steps 3 through 7 to clean the front portion of your car.
Place your car's wheels back on its front end, secure the lug nuts and jack up the vehicle to remove the jack stands. Lower your car slowly to the ground and tighten the lug nuts completely.
Every car contains sealant on its underbody that protects the paint from rusting. The sealant is shiny and long-lasting, but over time the sealant begins to break down. If your car is older and its underbody sealant is chipping or completely gone, purchase a can of spray-on underbody sealant and apply it according to the directions on the can -- after completing the cleaning process outlined in this article.
Never clean your car's underbody while the vehicle's brakes, engine, pipes and other components are hot --- wait until the components are cool. Washing the underbody while it is hot can result in the cleaning mixtures drying on the components.