Dark water stains on a knotty pine wood ceiling won't always come off with regular cleaning products and methods because the water penetrated beneath the surface of the wood. Oxalic acid is one of the best chemicals to remove dark stains from wood. It's often sold as wood bleach, and may be found at hardware or home improvement stores. Keep kids and pets out of the area while you mix the chemicals and clean the ceiling to prevent any accidents.
Put on rubber gloves and goggles before handling the oxalic acid. Open any doors or windows to ventilate the area while working.
Measure 2 to 4 tablespoons of oxalic acid crystals, depending on the severity of the stain.
Pour the crystals into a heat-safe jar and carefully add 1 pint of hot water. Stir it well with a spoon until the crystals completely dissolve.
Dampen a sponge or cloth with some of the oxalic acid solution and carefully apply it to the ceiling, standing on a ladder if necessary. Thoroughly saturate the stains with the solution.
Check the stains every 5 minutes to monitor the bleaching progress. If you leave the bleach too long, it will create lighter spots in the wood. Once the stains are sufficiently lightened, mix 1 tablespoon of sudsy ammonia in 1 quart of water and rinse the wood with a cloth dipped in this solution to neutralize the acid. Alternatively, use 2 tablespoons of borax mixed into 1 pint of warm water.
Dampen a clean cloth with water and rinse the wood to remove any residue.
Pat the ceiling with a dry cloth to remove any excess moisture.
Sand the cleaned area lightly with fine sandpaper following the direction of the grain to smooth out the surface if you plan on refinishing the ceiling.