Painting onto bare but previously painted plaster

first of all you need to solve the problem of ventilation in the bathroom, steam from showers and baths without it is causing these problems. strip all paint work back with sharp scraper and very lightly sand all walls/ceiling with fine sand paper (palm sander if you have one) then seal all areas with plaster sealer/anti condensation paint. apply only bathroom recommended paints..see your local paint supplier..not diy. always ask for advice when buying ..make sure it does exactly what it says on the tin......

Remove the bubbling paint, paint the bare plaster with mist coat. Fill over the areas where the existing paint was removed and touch that up once it's been rubbed down with mist coat then paint the room with zinsser permawhite. Its designed specifically for bathrooms as it an anti mould paint that resists cracking,peeling and blistering.

The other answer by another painter is fine but you will need to treat the mould using a anti mould spray which you can purchase at any good diy store or on line, then as the other painter stated scrape as much of the flaking paint then sand down and then apply a coat of primer sealer which you need to use to stop the paint edges from peeling again then apply a wipe able emulsion or you can look online to find some manufacturers make a mould proof paint but your limited to only certain colours.

All the above is sound advice for prep etc..The anti mould treatment is imperative also is the ventilation curing.Also to add if you don’t conquer the ventilation problem the mildew ,mould spaws will reappear so a bathroom paint ,diamond eggshell ,Zinzer etc is a must.These arn’t cheap but will last you and you can wipe clean.Take heed the preparation guide .Good luck ..