Oxalic acid is a wood bleach. Recommended for rust and damp spots and also to freshen up sun greyed timber. (Oxalic acid is sold under a variety of names as teak or deck cleaner for yachts) Disolve the powder in hot water until no more will disolve, about one part to eight by weight. Apply to the wood with a brush and leave for at least 30min and up to 3 hours. wash off with water. It's not highly corrosive but is an acid so be sensible both in storage and use. wear rubber gloves and goggles and wash off skin.