Cleaning Indian Sandstone

Our sandstone was laid around 7 years ago and is filthy with black spot and lichen. A jet wash will help, but I need much more than that this year. Seven cold wet winters has taken its toll.

I have read it needs washing with high strength caustic. The wife is nervous about the impact of run off to her garden plants.

We tried to use a weaker base (soda crystals) to protect the plants, but that was useless cleaning the sandstone. So, caustic it is.

Any experience of using caustic on sandstone and how your garden plants faired?

Also, we have some plastic grass, so anyone have any direct experience of impact of run-off reaching this, and any impact?

Any tips are useful.

Thanks in advance.