Here’s a quick tutorial on how to smooth the bottom of bisque fired pottery. Simply take a square of 80 grit wet-dry sandpaper and affix it to your wheel head with a glue stick. Hold your pot bottom to the sandpaper and spin. This will kick up dust, so you might not want to do it in a shared space. You should also wear a mask. You can sponge a little water onto the sandpaper to unclog it and to cut down on dust. When you’re done, just wipe off the wheel with a towel and it’s clean again.
Here’s a video I just came across on YouTube with some great pointers for sanding safely. Try dipping the bottom of your pot in water before using the wheel. I think this would be more effective than wetting the sandpaper on the wheel.