Round off the corners of your scraper—whether it's a wide putty knife or drywall taping knife—so it won't gouge the ceiling and leave you with dozens of ceiling wounds to repair. Use a file, a sander or an electric grinder to do this.
Scraping alone won't leave you with a paint-ready ceiling. You'll probably have small dings and gouges to fix. At a minimum, you'll have to sand the ceiling to get it perfectly smooth before painting.
Here are some tips on fi drywall and getting it ready for paint.