Discovering how to remove rust stains from wood introduces the hobbyist to oxalic acid. This powerful chemical is a bleaching agent that counteracts the chemical reaction culminating in rusting. In wood furniture--especially cherry, mahogany or oak--there is a heightened presence of tannic acid that reacts with iron and moisture to form dark markings and moisture rings that necessitate this treatment.
Remove the wood's surface patina. This includes wax build-up, dirt and furniture finish. Apply denatured alcohol or lacquer thinner--depending on the type of finish--with an old or disposable paintbrush. Allow the solvent to penetrate and liquefy the finish; rub off the softened finish with steel wool.
Mix hot water with granular oxalic acid in a plastic gallon bucket. Keep at a ratio of 50 to 100 ml of acid to 1 litre of water.
Apply the solution to the wood with a scrub brush. Cover the entire surface, not just the area where the rust stains or blotches appear. Let the chemicals dry completely.
Repeat the application if the rust stains do not noticeably diminish. Give the wood plenty of time to dry in between applications, but especially after the final contact with the oxalic acid.
Rinse the wood with distilled water. Let the wood dry and visually inspect the surface. Repeat the rinse if you see oxalic acid crystals or residue. Antique restorer Jeff Jewitt strongly suggests neutralising the acidic wood by painting on an alkaline solution consisting of 1 litre of water mixed with 30 ml of baking soda.
Sand the wood surface and refinish the piece. Use very fine sandpaper in the 150- to 220-grit range.
Cover the largest area possible with the oxalic acid. The mix not only removes rust stains from wood but also bleaches the wood surface. Applying the substance to the entire area protects the furniture from having a blotchy surface after refinishing. Wear protective goggles and gloves.
Test furniture finish in an inconspicuous spot before deciding on the stripping agent; using the wrong remover adds hours to the labour process or harms the wood. Do not allow the oxalic acid mix to cool down. Consider placing the plastic bucket inside a heated water bath to keep the mix at the hottest temperature possible. Remove rust stains from wood in a workshop or area that is well-ventilated. The odours generated by finish removers and also the acid mix can be overpowering and lead to breathing problems.