Unsightly black stains on a deck are caused by metal being left on a wet deck. In the photo you can see where a bolt, a bracket and even a nail have been left on the deck and they have left a faint stain behind. That big, black mess that you can see on the right-hand side of the photo has been left behind after some metal has been ground on the deck and the metal filings haven’t been cleaned off properly. Then when water gets on the deck, the metal filings and the tannin inside the decking boards have reacted to each other and created a black stain. This black stain is near impossible to clean off and when it is as bad as it is in the photo, you have no option but to sand back the deck.
To prevent this from happening to your deck, it is best to clean your deck daily making sure to remove anything metal from the deck. Even leaving other bits of timber on the deck can lead to stains which are hard to remove. Kwila timber is especially bad for this, as it is a wood that is heavy with tannins. Tannin is the sap inside timber and even though the majority of decking boards are ‘kiln-dried’, they still have a level of tannin in them.
One product you can use to clean the deck if you do have these black stains is ‘Deck Clean’, which is a mild acid. If you decide to remove your black stains by cleaning with Deck Clean, you need to scrub like you have never scrubbed before and you will have to repeat the process at least a couple of times. In some cases you will have no alternative but to sand the deck back.
For more information of on installing decking boards have a look at our “Installing Decking Boards” guide which is available from the Resource Shop.