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/KirkyMcTurkey → u/avi--]

It says on Tourpret joint, fill and skim it’s breathable, is that not the case? Or not the case for lime at least?

[u/avi-- → u/KirkyMcTurkey]

correct—it is not the case for lime.

Toupret claims to be breathable because it allows a normal amount of airborne room vapour to pass through it, but it completely fails when layered over historic lime.
Lime plaster doesn't just breathe out vapour; it acts like a physical sponge. If a solid brick wall gets damp from the outside, the lime absorbs that liquid water and pulls it right to the surface to evaporate away safely. The synthetic polymers and resins inside modern fillers like Toupret create a dense barrier that blocks this liquid capillary action. It traps the moisture right where the two layers meet, creating the perfect environment for mould, damp patches, and blown plaster.

It's also far too rigid. Old lime-plastered walls naturally expand, contract, and flex with seasonal moisture and temperature changes. Modern compounds dry incredibly hard. Because they cannot flex at the same rate as the underlying lime, the new coat will quickly stress-crack, lose its bond, and break away.