Google found me this thread https://community.screwfix.com/threads/high-temp-paint.91261/ the last-but-one post has an answer. They reckon emulsion should be good to 70C. Outdoor masonry paint must be able to survive on rendered walls in a suntrap (ours get too hot to touch at this time of year, so that’s over 50C). It survives well below freezing too.
We used to use 50/50 parafin grease on folks legs but it fell out of fashion because it’s flammable and the dear old folk practically live on top of gas fires in the winter
I was chatting recently to a German engineer who worked on designing the heat protective tiling inside these massive chimneys that have to stand huge temperature.s. because of a mistake in this tiling they lost the entire inside of the chimney because the heat breached the tiles. Cost the company millions
Having read through the sites Gregg kindly linked to and finding a few more it sounds like a massive ball ache. As mine is only patching a few damaged areas I might just call in a man to inhale through his teeth a lot and do the job with polyfilla while I’m at the shop buying his choice of biscuit.
F.U.C.K D.I.Y