Oxalic acid* is very useful for cleaning grey & weathered timber. It is more commonly known as grey deck cleaners (I.e: Preschem Grey Deck Cleaner) and can be used on all types of timber. It is recommended for cleaning blackened and weathered decking boards.
Why does timber get blackened and weathered?
Leaving timber out in the weather uncoated may cause the timber to be blackened and weathered.
Timber which has been previously coated with Linseed Oil or some other similar finish may require vigorous scrubbing with Oxalic Acid to clean off the discolouration. To clean grey decking boards, use either a grey deck cleaner or products containing Sodium Percarbonate, like Napisan mixed in a bucket of hot water. Apply the mixture to the deck with a hard bristle brush or broom, leaving it for around 15 minutes before washing it off with a hose. Depending on the state of the boards, it may require 2-3 washes do get the timber looking like new.
* Care must be used when using oxalic acid as the crystals and diluted solution are poisonous, gloves should be warn to avoid contact with the skin.