Mildew can permanently damage the waterproof coatings on your tent by causing them to separate from the fabric, but mild to severe staining is more common. To prevent mildew, it's important to dry your tent as quickly as possible after it gets wet. Even if your tent appears to be dry after use, it's best to assure it is completely dry before storing. Hang it outside or pile it loosely in your house for a few days, turning it inside and out to assure it has dried everywhere. Storing a wet tent for as little as 24 hours in warm weather can start the process of mildew formation. Never machine dry your tent as the heat can melt the fabric. If you notice mildew on your tent, you can use a solution of one part vinegar to three parts water to clean the affected area, then rinse thoroughly and let it dry completely before storing.