If you are lucky, you can savage a refinish job while the finish is still wet. Make a circle with masking tape with the sticky side out, and try to pull out the sag when it is still wet.
If your finish dries with the curtain in place, your only option is to WET SAND it out, using a BLOCK, after the finished has cured enough that it will sand, rather than gum up on the wet/dry paper.
