Several products are available for dealing with discoloration and stains. Commercially-available powder or liquid-concentrates have a base of non-chlorine bleach or oxalic acid; a detergent may be part of the formula. Bleach-based products eliminate mildew, acid-based materials handle graying and stains. Some products may darken woods such as redwood and cedar, so be sure to test any material in an inconspicuous place.

Always wear rubber gloves, goggles, and old clothes when working with these chemicals and follow the directions explicitly. Caution: Never mix detergent containing ammonia with household bleach; the resulting fumes can be highly toxic.

Test for mildew by applying a drop of undiluted liquid household bleach to a small, black spot. If the spot disappears after a minute or two, clean the deck with a mild cleanser (no ammonia) and rinse with a solution of 1 part household liquid bleach to 4 parts water, then rinse with water. For stubborn mildew, mix one cup trisodium phosphate (TSP) and one cup household liquid bleach in one gallon of water and scrub with a stiff bristle brush. Rinse after about 15 minutes.

Non-mildew stains are often caused by natural wood extractives or corroding hardware and nails. Familiar, general graying of the decking is usually the result of surface wood cells that have been broken down by UV radiation and wear. Bleach used to kill mildew can also leave a surface drab and washed-out.

For these problems, an acid-based deck restoration product is best. You can buy a pre-mixed oxalic acid deck cleaner or purchase oxalic acid crystals from a hardware store or home-improvement center and mix a solution of four ounces crystals to one quart water in a non-metallic container. Wearing rubber gloves, eye protection, and old clothes, apply with a rag to a one board at a time and scrub with a soft brush. Allow to dry, then rinse with clear water.

You can strip and clean a deck that has an existing stain finish, using a commercial "deck scrub" that you brush in and rinse off. Follow the label directions.