SCRAPE A STUCCO TYPE CEILING

A true "stucco" is made of cement and will be very difficult to scrape clean.  Sometimes what may look like "stucco" is in fact a drywall compound that is applied like a stucco finish.  If this is the case then directly sanding the surface down with a reduction of heavy-to-light grit sand paper will work.  

In my experience, I have found that applying a couple skim coats of drywall compound directly over the surface and then sanding to smooth - is a more direct way to achieve your goal.  Prime and paint when finished.