After having my bathroom ceiling replastered about 6 years ago to hide an old water mark the paint started to flake off not immediately but after a year or two,we had the chimney stack removed thinking there was water getting in through the old pointing and had the ceiling treated and repainted,now two years later its peeling again,we have brushed off all the flaking paint and sanded the whole ceiling and have now coated it with two coats of sika 1 waterproofing left it a day and have know tryed to paint it again only for the old paint to peel off as were rolling on the new paint...any advice would be greatly appreciated.
Plaster is tricky. In many ways (as you know from experience) it acts like a sponge and soaks up water. It can look nice and dry from the ceiling point of view but can still be holding moisture on the backside (attic). After a leak has been repaired it is better to wait 2 or 3 months to allow the area to dry out completely before repairing. I would almost bet that the leak is still there if it is again peeling. My only advice is that you go into the attic above the bathroom during a heavy rain and find out if it is indeed still leaking.