Remove plaster residues from sandstone

If there is a spot where you can't see, maybe try [Zementschleier Entferner], but that is an acid.

Or a high-pressure cleaner.

In the Neckar valley, there is sandstone everywhere, and where it had to be properly cleaned, such as on public buildings, it was sandblasted. Ultimately, you can also try a steam cleaner sometimes, but usually, the dirt is too deep in the porous stone. I don't think chemicals will work. I once had a wall that was smeared with cement; time took care of it.

A wire brush will probably cause more damage. I assume that there was a cement mortar applied there. To properly clean the stone, you usually have to remove material, that is, blast, grind, or carefully remove it by hand. I would use a sharpening stone, so not with a machine. It depends on how thick the mortar layer still is. We often glaze small remaining parts because otherwise too much of the stone would have to be taken away. Without photos, however, it is difficult to give a more precise recommendation.

Here is a photo of the object ...

Oh dear, I had completely different images in mind, here I would try power washing with the steam cleaner; if that doesn't help, then dry ice blasting.

Thank you!
Dampfstrahler = Hochdruckreiniger ?