Drywall guys often skim mud over the edges of electrical boxes and inadvertently fill the fixture screw holes. Before they can fill them, though, I fill them first. I use inexpensive wax toilet-seal rings to do the job. The wax is soft, and it’s easy to press a dab into the screw holes. Any compound that covers the box edge pops right off and leaves the holes free and clear. The wax also lubricates for easy screw entry.
Mike Guertin, East Greenwich, RI