I discovered a simple way to clean the rust stains that can occur on a popcorn ceiling from moisture. I misted 100% bleach on the area. In minutes the stain was gone. When the ceiling dried (about 30 minutes), the area I repaired blended in perfectly with the rest of the ceiling. The most difficult part of this task was the prep. I set up the area just as I would were I spray painting. I taped the crown moldings and used absorbant material (several layers of newspaper) rather than plastic to protect from overspray. Also, make sure to use a good sprayer or mister....one that doesn't drip (all over you!). I am going to try a similar process around the heat vents that have dust accumulation, using a soap....hope it works as well.