If you've ever been in the middle of a refinishing project only to discover that the wood you've just uncovered has black marks and stains, or if you have weathered cedar or redwood that you'd like to brighten up, then you'll want to learn about oxalic acid. Oxalic acid is a handy substance to have around the workshop, as it bleaches out many stains without bleaching the wood's natural colour.
Iron, water and urine Stains
Rust and trace metals in water or pet urine can cause black or blackish-blue iron stains on wood. Oxalic acid is effective at removing iron stains, but it may take three or four applications to totally remove them. The oxalic acid method works only on bare wood, so you must strip all stains and coatings before applying oxalic acid.
Weathering of cedar and redwood
Cedar and redwood contain naturally occurring resins called tannins. When wood containing tannins becomes wet, the tannin rises to the surface, where it causes black or brown stains. Oxalic acid is effective at removing tannin discolouration, but again must be applied to bare wood.
Large vs. small applications
If you've have a large deck or floor to bleach, you can mix 450 ml of powdered oxalic acid with 4.5 litres (1 gallon) of hot water, then scrub the solution onto the wood with a large scrub brush or push broom. For smaller areas, such as a water ring on a table top, mix 36 ml (2 tbsp) of powdered oxalic acid into 1 litre of hot water. Use a small paintbrush to apply the solution. These mixing instructions may not apply to the oxalic acid you purchase; read the label for proper instructions for your particular product.
Although fairly mild when diluted for wood cleaning, oxalic acid is quite corrosive at full strength. Always wear gloves and eye protection when working with oxalic acid, and only use outdoors or in a well-ventilated room.