Oak cleaning with oxalic acidJust hearing the word "acid" sounds ominous. But oxalic acid is easy to use and the safest for the home. In fact, it is found in many vegetables including spinach, and rhubarb, and is also used in shampoo and washing powders! Perhaps its best known use is that as a wood bleaching agent. Exterior wood tends to turn a dark grey after being exposed and Oxalic acid solutions are used to lighten and brighten dingy grey wood. In the refinishing of wood furniture, oxalic acid can be used after stripping to lighten and soft darker stained areas before refinishing.
A few other uses for oxalic acid are :-
Dark Spots, tannin stains & discolouration of the wood surface are usually caused by moisture and is exacerbated when the moisture is contaminated with metal such as iron and steel (especially on Oak & Idigbo) and can be removed with oxalic acid. Oxalic Acid can also be used to freshen wood colour if it has become lightly greyed due to external exposure to sun light. In the refinishing of wood furniture, oxalic acid can be used after stripping to lighten and soften darker stained areas before refinishing. It can also be used as a rust remover and a stain remover in many other applications.
Oxalic Acid is best bought in crystal form and looks just like sugar.
Simple to use, mix with warm water as you would mix sugar in tea, in a clean container. A mix ratio of 1 KG to 6 litres should suffice. Add the crystals to the warm water, stirring until it is dissolved, you then have a ready to use saturated solution, adding more Oxalic acid crystals will NOT make the solution any stronger. You could use Methylated Spirits instead of water. Using Methylated spirits allows for quicker drying, enabling a protective coating to be applied to the timber sooner.
Apply the solution directly on to the timber, and wash with a scrubbing brush. It's best to wear gloves too! Any surface coating such as wood stain, paint, wood oils etc will prevent the oxalic acid solution from working, so all protective coatings will have to be removed first.
It may require multiple applications to fully remove the staining but once removed the timber will need to be thoroughly cleaned with clean water or meths and then allowed to dry, in advance of any other potential coatings to be applied, such as a wood stabiliser or an Osmo oil.