How to remove black marks from my year-old deck?

[pongnz]

Last year I had a new treated pine deck installed. I waited a few weeks for it to weather and then stained it with Cabot’s. I am seeing a lot of black marks on the decking and it doesn’t appear to wash off. Only if I remove  the stain can I seem to get at the black marks. I guess my first question is what is causing it? Is it tannins, or mould? How do I go about fixing it and protecting from it in the future. I am a bit of a newbie but don’t mind a hard work to correct. Thanks for any advice.

 

 

 

[JacobZ]

Hi @pongnz,

Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.

When did you first see the marks appear? 

If it's only been in the last few months, then your coating may need to be refreshed as it's potentially a mould-type growth. Oil will prevent water from getting into the timber and causing mould growth, so recoating more regularly is likely necessary in future.

I'd suggest giving it a good scrub with Cabot's Everyday Deck Wash and a stiff-bristled broom like this Oates Long Handle Deck Scrub Brush. Once cleaned, give any remaining black marks a spray with Concrobium Mould Cleaner and give it another scrub. Wash everything down with clean water before proceeding.

At this point, if there are still black marks, you may need to sand back your deck to remove marks that have penetrated deeper. Check out How To Sand A Deck for some guidance on this.

Once the marks have been removed, you can reapply your stain/oil, following the manufacturer's instructions for application. You can also check out How To Oil Your Deck for some guidance.

Moving forward it is worth doing a six-monthly maintenance coat to keep it all sealed up.

Let me know if you have any further questions.

Jacob