Construction year 1954 - What type of plaster do I have and how do I maintain it?

Hello everyone,

I have acquired my parents' house and would like to know which plaster is currently applied. Hopefully this works via picture. According to a plasterer, it would be lime/sand. However, there is a thin layer of paint from that time on top. The questions I now have:

[*]If it is lime plaster, I want to preserve its good properties, but I need to repair/level some areas and would then apply painting fleece. Is that possible? Which filler should be used for this?
[*]Does the already existing paint impair the properties of the lime?

An offer is available: In doing so, one would first apply reinforcement and then apply the filler directly onto the fleece, without sanding, similar to filling painting fleece. Does the filler or the fiberglass fleece then hinder the lime plaster?

Since I have just replaced the windows with <1Uw, I would rather not have that preserved new build feeling, meaning the plaster should continue to act as a regulator.

Thanks in advance!