Oxalic Acid is recommended to remove Water and Black Metal Stains.
Black metal stains occurs when welding or grinding occurs next to the timber, bits of iron filings land on the timber where they react with the timbers tannins, causing the area to go black. Water stains are also another unpleasant sight on your new decking boards but can be easily removed like the black metal stains by using oxalic acid*, commonly sold under the name of grey deck cleaners (E.g: Preschem Grey Deck Cleaner).
To get rid of these stains, mix the grey deck cleaner in water (creating a 1:10 ratio of oxalic acid to 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.