A.Charlie Jourdain responds: To remove a mild case of mildew, scrub the surface with a mild cleanser or non-ammonia detergent. Then rinse with household liquid bleach to kill the mildew spores. Finally, rinse with water.

For more severe mildew infestations, scrub the wood with a stiff bristle brush, using a solution of one cup trisodium phosphate, one cup liquid bleach, and one gallon of warm water. (Wear rubber gloves!) After scrubbing, rinse with a solution of four ounces oxalic acid crystals dissolved in one gallon of warm water in a nonmetallic container. Apply with a soft brush to one entire board or defined area at a time. After the wood has dried thoroughly, rinse with clean water. This will remove not only mildew, but also any water stains and extractive bleeding. It might reduce nail stains as well, however, but nothing short of sanding will remove nail stains entirely.

Charlie Jourdain is a finishes specialist at the California Redwood Association.