Cover manufacturers recommend monthly cleaning for your boat cover. Begin by brushing off any loose dirt, and then hose down the cover with water. Next use a soft bristle cleaning brush to apply a cleaning solution made of soap and water. Don't use detergent, because it may strip off waterproof coatings. Take a brief break while the cleaning solution permeates the fabric, and then hose off the cover until the water runs clean. Let the cover dry completely, ideally in direct sunlight to reduce or prevent mold growth. Once the cover is completely dry and mold-free, apply a waterproofing finish.