ᐅ Is it possible to remove black stains and green algae from a sandstone staircase?

Hello everyone,
we have a sandstone staircase in the garden that I clean every year with a pressure washer. Probably not the best idea, but it’s quick, and what else can you do to remove the black stains from the steps and seal them?

Is it advisable to seal the sandstone base from the inside using drilled holes, as some companies recommend, to reduce the dampness in the basement?

I have had good experiences with sodium percarbonate.

How the method works

1. Obtain the powder
* Sodium percarbonate (often sold as “oxygen bleach” or “oxy cleaner”).
2. Prepare the solution
* About 30–50 g of powder per liter of warm water.
3. Apply
* Use a watering can, spray device, or brush to apply to the wall.
* The stone should be slightly dampened beforehand.
4. Let it work
* At least 1–2 hours, preferably longer.
* Then scrub once with a brush.
5. Rain does the rest
* Over the next few days, the stone usually brightens noticeably as organic residues continue to break down.

Doing this 1–2 times in spring makes the stones look like new.