Removing the Black Spots
The goal here is to create a mild cleaning solution that will rid the furniture of mold, mildew and debris without damaging the piece. In this case, a simple soap and water solution is perfect. Mix a gallon of warm water in a bucket with three squirts of liquid dish soap. This soap is mild enough for all outdoor furniture, yet strong enough to cut through mold, residue and buildup. Use a brush or cloth to scrub the solution over the furniture, concentrating specifically on the spotted areas.
For tough spots that are not easily removed, use white vinegar. This is a natural disinfectant that works well in killing and removing mildew, as well as removing any odor caused by the mildew. Mix 2 cups water with 1 cup distilled white vinegar In a spray bottle, then spray onto the black spots and scrub with a brush or cloth. Repeat until the spots have been removed, then rinse to remove any remaining residue.
After removing the black spots, it is important to apply a protectant that will seal the furniture and prevent future mildew development. When washing is complete, set the furniture in direct sunlight so it can dry thoroughly. Once dry, use a brush to apply one or two coats of a waterproofing agent, and let dry again. This will protect the furniture and keep it from absorbing moisture for up to two years, which should prevent black mold or mildew spots from forming during this time.