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How to remove black stains on timber deck?

Hi everyone,

Our rental property came with an outdoor decking area and last year, the landlord did an inspection and asked why there were black spots on the decking. We were unsure and so it was determined that it was our fault and that it must’ve been due to oil spilling off the BBQ (even though we hardly use much oil).

we tried cleaning it based upon the landlords recommendation with the product he recommended but it made it worse and didn’t remove the stain.

there are multiple black stains/patches across the decking, some of which aren’t next to the BBQ and so we are wondering if these black stains are from something else?

or if we are using the wrong cleaning method?

can anyone confirm as to WHAT these black spots are? HOW they occurred? WHAT can be done to rectify?

I believe the decking is made of Merbau?

see photo attached, thank you!

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Comments

MitchellMc

Welcome to the Bunnings Workshop community @zobarks. It's wonderful to have you join us, and many thanks for your question about deck stains.

These types of stains can sometimes be caused by tannin leaching, which is common with Merbau timber. Merbau is known for releasing natural oils and tannins, and if the timber gets wet or exposed to moisture, those tannins can seep out and create dark stains, especially when in contact with iron-based materials, such as nails or screws, or even acidic cleaners. When was the last time the deck was oiled? This should be done yearly, or it can become deteriorated and start to decay.

Another possibility is fungal or mould growth, which can cause black patches, especially in areas with high humidity or moisture. Mould could easily form if the decking is in a shaded or damp area. The white marks could indicate fungal growth.

The most effective way to clean the deck is with a product like Cabot's Deck Clean. It might take a few applications to remove the stains, and if that isn't successful, the deck would need to be sanded. After cleaning, the deck will need to be re-oiled.

To the eye, it does look like oil staining, but it's not really possible to give you a definitive answer as to what it might be. The white marks suggest something other than oil staining. It might be worth having a deck restoration professional assess the deck to determine the cause. That way you can pass it onto the landlord.

Please let me know if you have any questions.

Mitchell

zobarks

https://www.workshop.bunnings.com.au/discussion/comment/49286#Comment_49286

Appreciate your quick response Mitchell. We had been told by a maintenance man a few months back that it looks like it could be mould or leeching. So that’s consistent with a previous conversation.

in regards to your question about the white marks, those are caused by us after we were asked to clean the stain with a deck cleaning product. It lifted the colour of the decking when we used it so that’s how it occurred.

I’ve attached some more photos in case it’s helpful.

Thank you

zobarks

https://www.workshop.bunnings.com.au/discussion/comment/49286#Comment_49286

Sorry Mitchell, me again. When you say that to the eye, it looks like oil staining - are you referring to oil stains that occur naturally in the wood (tanning/leeching) or are you saying it looks like BBQ oil stains?

was just unsure which one you are referring to.

thanks

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/86726#Comment_86726

Thanks for the additional images, @zobarks. They show that the deck is very deteriorated and likely has not had maintenance coats of oil applied in the required timeframe. The coating is peeling, and the deck is now at a stage where sanding is required to remove the deteriorated coating, and a fresh coat applied. None of this is caused by you.

The black patches could be due to the deteriorated nature of the deck. Water has been able to access the timber, and these black marks could be a result of that. So, regardless of whether these black marks are from oil or not, the deck needs to be entirely sanded due to the lack of maintenance.

To the eye, the black stains in your first image do, on first impressions, look like BBQ oil drips, which is likely why the landlord has presumed that. Given they are all over the deck, it points to something else, like timber decay.

It's time for the deck to be sanded, and a coating should be applied to it to prevent further degradation.

Mitchell

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