When exposed to the elements, composite siding is prone to mold and mildew. Cleaning composite siding is pretty simple. You’ll need warm water, a high-pressure hose, a bleach solution, and a mild detergent. Use the bleach solution to kill mold and mildew, rinse it off, clean the surface with warm water and mild detergent, then rinse off the soap and let it dry. It’s best to clean composite sidings on a sunny day to dry faster.