Painting - Paint flakes off ceiling

I have justed started my new house and was paint ing the ciling when the paint on the ceiling came off on the roller. I was able to get a stripping knife underneath the paint and it came away in big lumps, easier than peeling wall paper. There was a white paint on the ceiling which sounds like plaster board. Some paint did not come away but I'd say 90% of it did. I sanded down the ceiling with medium sand paper and tried to paint over it after. The paint has flaked away around the areas that did not come away and also in other areas. I realise now I have to apply a polybond onto the ceiling to allow the paint to stick but do I need to take all the paint off, i.e sand it down bare, apply the sealer then apply the paint? or can i get away with applying the polybond over the current paint and apply the final coat of paint. Paint is just normal white emulsion ceiling paint. I'm a beginner to this so any help is apprecaited.

Confixit

I would try the polybond over the existing paint first. If that works, then apply the finish coat.