Best way to fill please ?

[Alexhenshaw]

So on my DIY travels, I have encountered this very awkward red and extremely flaky plaster. What would be the best product and method to fill gaps of this nature. This plaster has proven extremely awkward and seems to want to crumble even when looked at . Thanks for any help !

 

[PrenticeBoyofDerry]

I would soak it with PVA/primer, then once dried, another soak till going tacky, then probably for that small area use one coat, likely to need a couple of applications of it.

 

[vinn]

For a start, you should have cut the paint/sealer free before attempting to remove the skirting.
Scrape off, and sand the decorated finish before attempting to replace any skirting.

You appear to have a sand & lime mix render thats been applied in two thick coats and skimmed over.
Use any of the small bags or boxes of patching plaster or sand and cement, and follow the bag instructions to apply.