Beware of paint buildup
Decades of paint buildup can make a door rub against the jamb or door stop molding. The fastest way to remove paint buildup is with a sharp stainless steel or carbide scraper.
After scraping, sand the door to smooth the scraped edges. Use power sanders sparingly—high-speed sanding can melt paint, making it even more difficult to smooth out.
Use sandpaper rather than a scraper on metal doors. Chemically strip fiberglass doors if they have flaking paint—you'll quickly ruin a fiberglass door (smooth or wood grain) if you scrape or sand it.
If your home was built before 1979, check the paint for lead before you scrape or sand. Find more information, here.