Mysterious wall coating under old wallpaper

Hello everyone,

I am currently renovating my grandparents' old house. When removing the wallpaper, a strange wall coating was revealed. Thin and superficially very smooth. Does anyone have any idea what this could be? It comes off with a spatula. If anyone has seen something like this before, I would be grateful.

Yes, we had seen that during our renovation as well. It was on our ceiling. We painted everything with concrete contact primer and then replastered it, and otherwise didn’t stress about it :-)

is probably lime sludge. depending on adhesion to the substrate, it can be left on

a strange wall coating

I simply think that is wallpaper paste.