Does anybody know of any product that can be used to prevent mold and mildew from forming on outdoor furniture

cushions? The cushion covers are of a removeable heavy weight cotton fabric over a foam type material covered in batting. The furniture is kept in a covered, screened in area which does get damp when it rains. I have removed the covers and have washed them with detergent and bleach but after a couple of weeks the mold and mildew return. I was wondering if there was a product out there that can be sprayed on like a scotch-guard type product which will prevent and/or protect the cushions from the mold and mildew?