Painting a Chalky plaster Ceiling

[belvir]

I'm re-painting one of my Bedrooms and the existing paint fell off the Ceiling.
Looks like the pure white (plaster) material just on the Ceiling, I sanded it down a little but is still
very powdery when I brush it. Do I need to seal this? I don't want my new paint flaking off.
If I do, what would one recommend, and how many coats?

[Lappa]

How old is the house?
The ceiling may have been painted with kalsomine/calcimine which is a tinted pale “wash” used on walls and ceilings.
If so, it’s water soluble so use warm water with sugar soap in it to wash of the majority of it. The surface will then need to be painted with a binder to lock any remaining to the surface so you can paint.
If you just paint over kalsomine/calcimine the paint will just peel off.

To test for it, just wipe a damp finger on the surface and you will get a chalky residue on your finger.

My whole house was painted with it so I’ve been through the labour intensive process of getting rid of it as the previous owner had painted over it. All the walls and ceilings were scraped back ( large sheets of paint just peeled off), thoroughly washed, painted with a binder then painted with undercoat and then colour.

Zinsser make a good product called “Peel Stop” which is a clear binding primer.

[Beardy]

Good advice from Lappa, you can also use bondcrete or cemetik as a sealing agent but I think I would be using the Zinnser type products these days
Sonetimes if the ceiling is in poor condition it was cheaper , easier and foolproof to just skin the existing ceiling with new plasterboard