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just get some mortar and it should be fine
This is where I'm a bit wary - previous builders have mixed and matched modern with old materials willy nilly, and there are problems everywhere with damp because of it.
And although this is on an internal wall, it is an internal wall where damp exists, and I don't want to exacerbate it.
I may end up stripping of the top layer of gypsum plaster (which looks like it may come off in one piece) and giving it a finishing coat of lime plaster.
Which means, I guess, I just need to fill the hole with lime plaster & hair mix, then finish with a rough skim coat.
NotThamesWater-
I doubt that your damp problem is caused by the mixing of different materials.
There could be many different reasons for it.
But that is not one I've heard of. -
The only way to 'fix' this is hack off all the newer stuff, let the lime dry out and then re lime.
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If it’s a potentially damp wall, then don’t fill with bonding plaster as it’s very hygroscopic. Use a cement mortar mix which will probably be fine, or add a little damp proofing agent if it really has an ongoing damp problem. Build it up in layers if it’s over say 30mm deep, and leave the top to be finished with finishing plaster.
wildwest
lynx
rodabod
That will be too deep for polyfila just get some mortar and it should be fine.
Hard to tell how deep it from the pic. Most builder would use grip fill or expanding foam if the client is not present. And skim it jobs a gooden mate.