ᐅ Removing plaster residue from sandstone

If you have an area that is not visible, you might try using a cement residue remover, but keep in mind that it is an acid.

Alternatively, a pressure washer can be used.

In the Neckar Valley, sandstone is prevalent everywhere, and where it needed thorough cleaning, such as on public buildings, sandblasting was used. Ultimately, you could also try a pressure washer, but usually the dirt has penetrated too deeply into the porous stone. I don't think chemicals will work.

I once had a wall that was smeared with cement; time eventually took care of it.

A wire brush will probably cause more damage. I assume that there was cement mortar applied there. To properly clean the stone, it usually needs to be removed by abrasive methods such as sandblasting, grinding, or carefully removing by hand. I would use a grinding stone, not a power tool. It depends on how thick the mortar layer still is. We often glaze small remaining parts afterwards because otherwise, you would have to remove too much of the stone. Without photos, it is difficult to provide a more precise recommendation.

Here is a photo of the building ...

Oh dear, I had completely different ideas in mind. Here, I would strongly recommend trying a steam cleaner first, and if that doesn’t help, then dry ice blasting.

Thank you!
Steam cleaner = pressure washer?