For routine maintenance of sheet metal siding, rinse the walls regularly using water and a sprayer from a garden hose. For a more thorough cleaning, mix 1/3 cup of laundry detergent in a gallon of water, and scrub the siding using a long-handled brush with soft bristles, and rinse the surface with clear water. The process removes dust, dirt and slight oxidation from exposure to the sun.
Sheet metal siding with a baked-enamel finish is naturally resistant to mold or mildew, but in wet climates or with surfaces that are densely shaded, some growth may occur. Remove the growth by mixing 2/3 cup of trisodium phosphate, 1 quart of bleach and 1/3 cup of detergent with 3 quarts of bleach. Scrub the surfaces, and rinse the solution away.
While manufacturers recommend avoiding strong solvents for cleaning sheet metal siding, the use of a rag soaked in mineral spirits offers a safe and effective way of removing tar, caulking, wax, grease and oil. After removing the substance follow with a detergent cleaning and water rinse.
Repair scratches in the surface by sanding and repainting the affected area. Repair dents using methods similar to those for restoring damage to auto bodies. Replace any panels where the damage is too extensive to repair with two-part auto body filler.