How to stop Merbau deck colour changing?

I have my merbau deck installed around 3 month ago and stained immediately after completion, it rained during the installation. Recently the colour start changing drastically, below photos. Is it mould or tanning bleed. Shall I clean and scrap the existing stain and leave to weather.

Thank you

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EricL

Hello @mzaaroura

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your deck.

It sounds like your merbau deck is experiencing tannin bleed rather than mould, especially since it rained during installation and was stained immediately afterward. Merbau is known for releasing natural tannins—reddish-brown pigments that can leach to the surface when exposed to moisture, particularly in the first few months. If the timber wasn't given enough time to weather and release these tannins naturally before staining, the stain can trap them underneath, leading to dark streaks or uneven discolouration as moisture continues to draw them out.

This is quite common with new hardwood decks. Although mould can appear similar, mould typically has a powdery or fuzzy texture and is more likely in areas that remain damp or shaded. To check, you can do a quick test: dab a small area with diluted bleach—if the mark lightens, it’s likely mould; if not, it’s probably tannin bleed. The best course of action now is to clean the deck thoroughly using a deck cleaner specifically formulated for tannin and stain removal, the store stocks products like Cabot’s Deck Clean which will work well for this.

After cleaning, let the deck weather for a few weeks to allow any remaining tannins to leach out fully. Once dry and stable in colour, you can lightly sand the surface to remove any lifted stain or rough patches, then reapply a fresh coat of decking oil or stain if needed. Leaving the deck untreated for a little while after cleaning will help ensure the stain adheres better and lasts longer next time.

Here are some handy step-by-step guides:

If you need further assistance, please let us know.

Eric

mzaaroura

https://www.workshop.bunnings.com.au/discussion/comment/51732#Comment_51732

Thank yous so much, one last question, shall I use Cabot new timber deck product as stronger product to strip stain or Cabot deck clean will be enough

Thanks

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/88018#Comment_88018

Hi @mzaaroura,

I'd recommend the New Timber prep, and it will likely take a couple of applications and a decent amount of scrubbing to remove the staining. Failing that, you may need to lightly sand the stained area to see if the stain can be removed.

Please let me know if you have any questions.

Mitchell

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