How to remove black marks from Merbau decking?

[kj1]

Hi, my deck had these marks appear over night - how can I get rid of them ? 

 

[EricL]

Hello @kj1

Welcome to the Bunnings Workshop community. It's terrific to have you join us, and thank you for sharing your question about removing black marks from your Merbau decking.

Looking at the picture it seems something was dragged across the top of your deck. I suggest trying out the traditional method of water, dishwashing soap and a Sabco Bamboo All Purpose Heavy Duty Scrub. I recommend giving it a thorough scrub to see if the marks get lifted off. Please remember to scrub in the direction of the grain and in a left to right motion do not scrub it in a circular fashion.  

The second method is using a pressure washer. Please note that too much concentration of the pressure in a single spot will wash the oil and stain off the deck and leave you with an exposed timber spot. If your pressure washer has a low setting I suggest starting there. You can combine the first cleaning method with the pressure washer if the marks are tough to get off.

Unfortunately, most of the cleaning chemicals tend to wash out not only the dirt but the oils and stains in your timber decking. A bleaching effect occurs and one part of your deck will be lighter in colour than the other.  

Let me tag our experienced members @Noyade, @TedBear and @Adam_W for their recommendations.

Please keep us updated with your progress, we look forward to seeing your deck nice and clean.

If you need further assistance, please let us know.

Eric

[Adam_W]

Hey @kj1 and welcome!
Okay, that's kind of odd.
One area looks like something has been dragged, another like something has been left on the deck & 'imprinted', like the printing coming off a plastic bag that was wet underneath (I speak from experience...) there also appears to be part of a sand-shoe tread print there. Match the print to the shoe and you have the culprit!
But seriously.

It doesn't appear that the deck itself is scratched. more that something has left residue on the timber. With the wet weather around the country mould is becoming a real issue & can appear very quickly. Moulds will most likley & most readily appear where there is something for them to establish on. A spilt soft-drink for example.
Might be a bit of a trial & error process. @EricL 's suggestions are good however I tend to only head towards hard mechanical cleaning after other options have failed as with pressure washing it can leave marks on the timber.
There are some very good special purpose deck cleaning products that may be worth trying.

This one is a deck wash, designed for regular use, so not too heavy duty. While this one is designed for restoration and stain removal.
Personally, I'd be starting at the low end and working up if that isn't doing the job.