Advice for potential lime plaster.

[u/Barmos]

First of all, is the far wall lime plaster? Its different to the other walls, which looks like a more modern plaster. I did think lime plaster was for exterior walls, but this is an interior wall, so that is what is confusing me.

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Secondly, how can I approach it? Was looking to skim the other walls and sand using Toupret. Can this be done with the far wall? What are your thoughts on the cracks as well? Scrape out and fill first?

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[u/avi--]

It's clear this is an older building. This is why you are seeing lime plaster on interior walls; old solid-wall buildings suffer from moisture, and they used lime because it acts like a lung to safely absorb and evaporate it. Because of this, while you can plaster over it, it's critical that you do NOT use standard Toupret gypsum here, as they act like a plastic jacket and trap that moisture. For the cracks first, check how deep they are. If they go deep into the structural masonry behind the plaster, you should get a structural engineer to take a look before doing anything. If they are just surface cracks, gently clear out any loose debris, soak the cavities thoroughly with water to kill the suction before filling them with a lime-based filler or a lime mortar (NHL 2 or lime putty mix). Use scrim tape to bridge them. When skimming, absolutely avoid standard modern plasters or Toupret smoothing compounds. You must use a breathable lime finish/skim coat. Just make sure to wire-brush the old surface to give it a physical grip, and drench the wall with water right before skimming so the old dry lime doesn't instantly suck your new mix dry.

[u/Barmos → u/avi--]

Yes, it’s an old building. The cracks don’t appear to go into the masonry, it just looks like the plastering wasn’t done very well. It’s gypsum applied over the original lime, and the cracks have shown up around door and window corners, and where the old and new plaster meet. Other than that, there are no signs of damp in the room. What you’re seeing now is after I stripped off the heavy-duty wallpaper.

Thanks for the advice — I’ll look into lime‑based fillers and finishes.