Letting your wet sails stay furled or flaked in a sail bag creates the perfect incubator for mold and mildew to grow. All sail cloth types, when stored wet, will promote the growth of mildew, but it’s really a problem with less breathable laminated sails. Keep in mind, where you store them is also important. Damp locations also attract mold and mildew. Regardless of how it gets there, once you have mold, your sail needs a professional cleaning.