How to fix black patches on Merbau decking?

Diana06

I’m after some advice on how fix these black patches on my merbau deck.  Deck measures 5x5m. The deck is 2 years old and has been sealed twice in that time with Intergrain Ultradeck. I am questioning the integrity of the 2nd coat as I don’t believe it was applied properly by the person we hired. The deck is uncovered and gets full sun for most of the day. I thought it was mould so used Intergrain Mould Remover followed by the Intergrain Deck Clean.  I used a pressure washer to rinse off both times.  In some of the places where the black patches were the stain has come off completely.  In other places on the deck the decking oil became slimy looking and looks to have completely come off.  If you zoom in on the photo where the deck is wet and black board you can see what I mean.  The deck has also started to bleed tannin as well in places. I noticed the black around October which seems to have gotten worse with all the rain in Melbourne. Not sure if the black is mould or tannin bleeding, Something else? Appreciate any advice on how to fix this.

MitchellMc

Welcome to the Bunnings Workshop community @Diana06. It's wonderful to have you join us, and many thanks for your question about preparing a deck.

From your photos, it looks like the black patches on your merbau deck are more likely timber degradation rather than just mould. The slimy areas where the previous coating has come off suggest that the second coat may not have adhered properly, which allows moisture to sit on the timber and exacerbate staining and surface breakdown.

At this stage, a good approach is to give the deck another thorough cleaning using something more aggressive than your previous cleans. Products like Cabot’s Timber Prep are designed to strip tannins and prepare timber for recoating, which should help remove much of the blackening. Be prepared to scrub well and rinse thoroughly.

If repeated cleaning doesn’t remove the black patches, the next step would be to lightly sand the affected boards to remove the degraded timber layer and any remaining coating residue. Once the surface is even and clean, you can reapply a quality decking oil or sealer, ensuring it’s applied evenly and fully absorbed to prevent similar problems in the future. This combination of cleaning, sanding if needed, and proper recoating should restore the deck’s appearance and longevity.

Please let me know if you have any questions.

Mitchell