HELP! Patio slabs need miracle

[Smileysmile]

I've attached photos of my jet washed patio slabs that are only 5 years old.

What are the markings that I can't seem to get out.

I've a powerful Karcher jet wash.

I've not spilt anything on them.

What is the solution? The best sandstone cleaner. Thanks

 

[Andydevon]

You could try this stuff

Patio Black Spot Remover For Natural Stone

For use on Indian Sandstone, York Stone, Limestone, Travertine (including Portland and Bath Stone) and all other types of natural stone.4 Litres: Maximum Area: 20 square metres.2 Litres: Maximum Area: 10 square metres. Patio Black Spot Remover is currently the only product developed specifically...

patioblackspotremoval.com

I use a 20% sodium hypo-chloride solution. Sold as a diary disinfectant at agricultural merchants. Above a bit less drastic and easier to use.

 

[Lower]

A more powerful jet wash will get the spots off. I get the same black spots on my Indian sandstone patio.