A.Robin Raymer of Plasterzone.com, an author, educator, and veteran plastering contractor, responds: How you treat the water stain depends on the type of coating on the ceiling. Unlike smooth-coat plaster, which must be finished with primer and paint, sand-finish plaster can be either painted or left its natural color. Or it can be tinted before application, a trend that seems to be increasingly popular.

If your sand-finished ceiling has not been painted, you can try to bleach the stain out. Mix one part water to one part bleach in a mist bottle and lightly spray the stained area until it's wet but not dripping. (Be sure to protect everything underneath, and wear skin and eye protection.) I usually spray in the morning and then let it sit till...

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