Repair of flaking plastic ceiling in bathroom over tub area.

Flaking plastic? Or is it the paint that is flaking off? You most likely will need to scrap off any loose / flaking stuff and patch / prime and re-paint it. If it is really bad you might need to replace the entire ceiling with some new drywall (grren board). You should replace your exhaust fan with a more powerful unit and let it run for a good half-hour after using the shower or bath to avoid moisture problems in the future.

If your going to go the scraping route... After the area is all scraped, use some KILZ and paint thewhole ceiling.This will help to seal up and prevent the already scraped areas to peel some more.ALmost acts like glue. Then take some drywall filler and smooth out the rough areas, prime , paint and finito!