How do i get the black spots off the grout between slate pavers?

[gnicholson]

[EricL · Accepted solution]

Hello @gnicholson 

Thanks for sharing your question about removing the black spots on your external grout. I suspect that they are deep seated mould and have started to anchor colonies on your grout. I propose using 30 Seconds 5L Outdoor Cleaner Concentrate, it is a quick and cost-effective exterior cleaner, used to restore those dangerous slimy green and slippery black surfaces to looking like new again. Also ideal for the treatment of mould and mildew on exterior surfaces killing the spores and minimising re-growth.

I also suggest using Oates Long Handle Grout Brush, its stiff bristles are sure to scrub off any mould or stain invading your grout. 

Please remember to wear personal protection such as gloves, goggles and mask when using the cleaner. Make sure to follow all the safety guidelines printed on the back of the bottle.

If you have any other questions we can help with, please let us know.

Eric

[Jason]

Do you have a pressure washer @gnicholson?

We have a crazy paved front verandah and simply use the pressure washer regularly to keep the mould at bay.

You do need to be careful though that you don't damage the stone or grout. Start gently, not too close...

Jason

[gnicholson]

Yep done that and also scrubbed thought someone might have another solution, this is on slate steps.

[gnicholson]

Thanks will give it a go. Have pressure washed and scrubbed and still there.

[MitchellMc]

Do you have any idea what the black spots are, @gnicholson? Are they some type of algae or mould, or perhaps they are oil stains?

As mentioned by @EricL, there's a good chance that they're impregnated stains in the surface and scrubbing might prove ineffective. If a solution like 30 Seconds isn't working, this might be a case of the stain being so engrained that it's not possible to remove it.

Mitchell

[gnicholson]

Definitely not oil but algae or mould as on places where oil would never be, just in two spots mainly so am buying that stuff to try today. Will let you know how it goes.

[gnicholson]

That didn't completely work so ended up using an angle grinder to get the bad parts off and out then filling the spaces as someone recommended before.

There is a grout saw but thought not worth spending the money for that for a small job.

Impossible to get a colour match after 16 years of weather, have done our best.