I've just phoned Crown to query use of the paint in bathrooms. They say there won't be any health risk but it's obviously not a bathroom paint & might not be up to the task.
The decorator is coming back in the near future to do ensuite ceiling & coving (which is bad for peeling) & I just know he's going to suggest the same paint. What would you expert's recommend instead? Zinsser perma white? Dulux bathroom paint?
Thank you for doing that. It will help others who come across this thread. I freely admit that I might have been mistaken about biocide levels. I am aware that exterior paints have additional ingredients, such as those which help them to absorb radiation (read: energy) from UV light. Ultimately though, exterior masonry paint is not designed to deal with steam, water, yes, steam no.
Perma white might be OK but if the substrate has not been prepared properly, it may well fail. Ceilings in rooms with showers often have a thin film of sower gel/soap particles on the ceiling. That is the stuff that the bacteria clings to (before becoming black). The advice offered by Burnerman has been spot on. I suspect that your decorator didn't sand anything, and didn't wipe anything down.
Apropos how far you go to prepping the area, it is tough call. BIN is great but not so great at dealing with steam. Getting back to Burnerman's advice, I would sand things back. I would additionally use a very, very sharp chisel to tidy up the edge between the ceiling and tiles. Whether to use silicone or not (to fill the gap where the ceiling meets the tiles) is a matter of contention. Silicone is fine if you never, ever plain to paint again. Silicone repels water based paints, and to a lesser degree (but a big degree) oil based paints. Faced with that, I would use a MS polymer such as CT1 to seal the gap. That said, you want the bead to be minimal. White CT1 will yellow in doors (they don't tell you that when you buy it). If you pint over it, it will be fine. Regular caulk may, or may not shrink and crack, and the unpainted edge will become black in time (if touched or in an area of high humidity).
Ultimately, you paid for someone to do a job, a job, which fialed pretty quickly.
Sorry, I don't know what to say. If you were one of my customers (even though I happen to be a decorator) my advice would be to accept the financial hit and find a decent decorator. If he becomes combative, please point him towards this forum. I will be happy to ask him questions about his work process.