Hi Zubar, as buildersmate says if the ceiling is that bad maybe it would be quicker and cheaper to get it reskimmed. That way you would have good surface to work with. As for the other with the paint flaking if it is a very old ceiling it could be what they call ceiling white under the emulsion and this will cause it to flake and peel. The only way to stop this is to remove all the ceiling white first. If you do have this on your ceiling reskimming may not help as the weight of the plaster will probably bring it down. You will know if there is ceiling white on the ceiling, where the emulsion has flaked or peeled just wash it and it will come off leaving bare plaster. As for sealing the stains I have always used a mix of 50% white gloss to 50% undercoat mixed together well, paint it on, allow to dry then a couple of coats emulsion will give good finish. The mixture of the 2 allows the emulsion to adere to the surface.