Rust stains on a sail can be a frustrating problem. Fortunately, these stains are easy to treat. Any type of acid will dissolve rust on a sail. The major consideration when cleaning a sail is the type of fabric the sail is made out of and the depth and age of the rust stain. Sails made out of natural fibers are easier to clean, while sails made of synthetic material are resistant and require stronger acidic solutions to clean thoroughly.
Take the sail down and spread it flat.
Scrub the stain with a rag, mild detergent soap and water to thoroughly clean the area.
Clean newly-made surface stains on sails with baking soda. Create a thin paste by combining baking soda and water. Dip a soft-bristled toothbrush in the solution and scrub the stain out. After you scrub, wash the area with a mild detergent soap and a clean cloth and rinse the area thoroughly.
Dip a soft-bristled toothbrush in rust-remover gel or lemon juice and stipple the solution onto any embedded stains. Allow it to sit for 15 minutes, then scrub the stain clean. Wash the area with soap and water and rinse it clean. Mix 1 tbsp. of baking soda in 1 qt. of water and sponge the area with the solution to restore the pH balance to the sail's fabric. Rinse the baking soda off with water.
Remove stubborn stains on sails made of synthetic fabric with oxalic acid powder. Mix the powder with hot water at the rate dictated by the manufacturer (the usual rate is 1 oz. of oxalic acid per pint of water). Apply the solution to the stain and allow it to soak for 30 minutes. Dip a soft-bristle toothbrush in the solution and scrub out the stain. When it is gone, wash the area with mild detergent soap and water, then rinse it clean. Mix 1 tbsp. of baking soda in 1 qt. of water and sponge the area with the solution to restore the pH balance to the sail's fabric. Rinse the baking soda off with water.
Take care when cleaning TemperKote and Mylar sails. Not much is known about these new materials, and acid may damage the fabric over time.