What exactly are we looking at? Is it a plaster and lath ceiling, or drywall? What exactly is that chip - just the paint layer thats cracked/ peeling to reveal bare plaster layer below ? Size - how big is the area in question?

You are looking at the 1 ft x 1 ft ceiling area affected by water leak. It is resolved now and I would like to patch it up.

I believe it is a plaster ceiling. I am not seeing lathe so I believe there is still a layer of plaster underneath that chip.

Try to take a better photo with more light and something in the frame for size reference to help us understand what we’re seeing. How large and deep is that ”chip”?

I would just scrape off the chipping and whatever else is loose, then wash/vacuum surface to be sure dust and grime is off (tsp substitute or any good pre-painting cleaner), dampen the bare plaster and apply either easy sand 90 or real finish plaster to get level with paint surface, then sand smooth, allow to cure (real plaster 30 days) .

I find real finish plaster to be less of a PITA in some ways than the drywall compound .... and thats what you would need for deep cracks and such.... so if you have a lot of original old plaster in the house that you anticipate repairing you might want to get that, along with a plaster+ concrete bonding agent that is painted on your old plaster to help adhere new plaster to old. I have several Zinsser primers on hand - Gardz is one that is good for new plaster/drywall compound, but just be sure to read label of any other thing yo ure considering cos not all primers are the same.

Spoze you could just remove chips, clean, prime and paint if you didnt mind the paint edges being slightly higher than the bare plaster. Honestly if its an out of the way ceiling in a n old house with a lot of other quirks... it probably doesnt matter!

Watch a few YouTube videos please.