Some of my boards on my merbau deck go black / very dark when wet. It’s freshly stripped and lightly sanded 120 grit, it’s mainly the ones around the edges where there is the most weathering as the old coating degraded.
Should those ones be sanded again? Would a deck cleaner / prep fix that? In a natural oil is a clear or merbau tinted oil better for a more uniform finish?
Hello @Sully1834
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your deck.
We've had similar questions about this phenomenon, I suggest having a look at this discussion - How to remove black stains on timber deck? by @zobarks. My teammate gave a brief explanation about why it happens. 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.
The suggested solution was to use https://www.bunnings.com.au/cabot-s-1l-deck-clean-1l_p1521451 . Another member also used https://www.bunnings.com.au/diggers-2kg-rust-and-stain-cleaner_p0960276 which was very successful.
Here are some discussions regarding removing stains on Merbau decking:
If you need further assistance, please let us know.
Eric
Hi @Sully1834,
Do these stains stay there even after the deck has been dry for several days? Or, do they only appear after it has been wet? The last four images just look like a wet deck that's recently seen rain. The mottled look is the timber drying at different rates.
In the first two images, the staining looks quite dark and potentially this could actually be staining. However, if it disappears once the deck is entirely dry, then they are not of concern and you can oil your deck. Merbau tinted oil will always be the best option for a uniform finish. The colourant helps blend inconsistencies.
Please let us know if you have any questions.
Mitchell
I put lots of photos is for a comparison as the boards around the edges go really dark when lightly wet and the ones more undercover stay a lot lighter and just get more of a shine effect. All the black parts do come lighter again once dry but not the full same colour as the others
I would say that the boards that remain darker have just been exposed to more weather and deteriorated faster, @Sully1834. When timber deteriorates, the pores in it open up and they absorb more water. It's the very earliest signs of decay. It's hard to say whether these areas need further work, or whether even sanding would break through the staining to fresh timber underneath. Cleaning them again certainly won't hurt, and it might just be a matter of doing a test section to see whether they blend in sufficiently with the surrounding area. I'd be inclined to just wait until the deck is completely dry and then oil it. If there's a bit of stain remaining and you're not happy with the look, you can always try re-sanding that area next year. I think the main thing is to get a layer of protective oil on the timber as soon as you get some dry weather, so the timber isn't further affected by the moisture.
