Iron staining is caused by a chemical interaction between iron and certain acidic timbers producing an unsightly blue-black staining within the wood in the presence of moisture. It is not strictly a fault of the finish although it can be exacerbated through inappropriate coating selection. The problem commonly arises around ferrous fixings such as nails or window furniture. Acidic timbers such as Western Red Cedar or Oak are particularly prone to iron staining. It is usually associated with translucent or semi-translucent stains which do not conceal the disfigurement and which are not as effective in controlling moisture uptake as opaque coatings. Disfigurement from iron staining can be removed by sanding back to bare wood and treating the discoloration with a solution of oxalic acid (NB oxalic acid is toxic and should be used with care - see below). Care should be taken never to use wire wool on wood as this can leave small particles of iron embedded within the surface which can create disfigurement in the form of black speckles or spots.
Oxalic acid can cause burns to the skin, although, as a weak acid the burns are likely to be less serious than those caused by strong solutions of mineral acids. Oxalic acid may be harmful if you swallow or inhale it.
Safety glasses. Gloves (e.g., nitrile, butyl rubber, neoprene or PVC) are suitable. PVA gloves are not suitable for handling oxalic acid or its solutions.
Small amounts of oxalic acid can be flushed down a sink with a large quantity of water, unless local rules prohibit this. Large amounts should be neutralised before disposal.