Plywood boards often have to sit in locations where they must wait for people to use them. Plywood is often stored outdoors, and may encounter rain, humidity and other conditions that create mold spores. If mold gets onto plywood, there is no need to throw the board out. You can kill mold spores and remove them from the surface of the wood with a simple bleach solution. This project will take several days to complete, because the wood must dry out between solution applications.
Wear old clothes that you do not mind getting bleached as you work. Put on the gloves and safety goggles. Mix a solution of 1 part bleach to 3 parts water. Place this solution in the small bucket.
Dip the brush into the bleach solution and scrub over the moldy area of the wood. Allow the bleach solution to saturate the wood. The wood may not appear cleaner at first, but after allowing the solution to sit for about 30 minutes the mold should start to disappear. Mold marks may not completely disappear, but the spores will die, preventing the mold from spreading.
Hose down the entire plywood sheet. Allow the wood to dry for 24 hours.
The bleach will cause the grain of the wood to rise to the surface, and you will have to sand the board smooth once more. Sand the surface of the board until it is smooth, first with rough grain sandpaper, then with a finer grain.
Dip an old rag into the paint cleaner and wipe over the entire surface of the board. This helps remove any remaining residue from the mold, bleach and water. Allow to dry for two hours.
Wipe a water-repellent preservative over all sides of the board. This will help the wood to resist further water and mold damage and make it perform better if used in an exterior setting. Allow the preservative to sit on the wood for two days before painting or staining the surface of the wood.