You can't just apply a layer over a water stain, as it will seep; you must first use a water- or oil-based primer. A stained area that is soft or flabby to the touch means that the drywall has suffered an impact and must be replaced before painting it, Lacroix points out. Paint the damaged area with two coats of oil or alcohol-based primer. Use a brush or roller and allow the stain to dry between coats.
This seals in any residual moisture and prevents the stain from penetrating. That's why it's so important to find and repair the cause of the water stain as soon as possible, even if it's no longer wet. Alternatively, it could be a one-off water spill or increasing humidity that leaves an unattractive and unwanted mark. This is most effective on white or light-colored walls and ceilings, and is a good idea if water stains start to appear on the ceiling or wallpaper stains.
Choose a paint that matches the color of the ceiling for a uniform appearance, then apply it to the primed area with a roller (choose an 8-inch nap cover for a smooth roof, or a ¾ inch to 1 ¼ inch layer for a textured ceiling). However, Bob Vila recommends searching for and repairing water leaks first, or you'll find that water stains on the roof continue to appear and repairs will be short-lived. When a water stain comes into contact with latex paint, the stain dissolves in the wet paint layer as the paint dries, causing the discolored mineral content of the stain to reshow through the paint through the paint to the ceiling surface. If any of them are present, thoroughly clean the surface with a dilute mixture of water and bleach with chlorine to remove it.
This is a good time to apply a few new coats to the entire ceiling or wall, testing different types of paint and finishes for the best results in your living room. Make the necessary repairs or seek the help of a roofer, HVAC professional, or plumber, as needed, and you can prevent future leaks and water stains on the roof. Obviously, the water will keep coming until the leak is fixed and you don't want the water to mess up a new paint. Do not start painting a ceiling while it is still damp, otherwise you run the risk of further damaging the existing paint and, possibly, the roof itself.
Whatever the reason for water stains, you need to identify and treat them before you even think about painting. Cleaning the stained area of the ceiling with a mild household bleaching solvent (one cup of bleach and three cups of warm water) will discolor the stain and remove persistent mold, grease, dirt or dust that may prevent primer and paint from adhering to the ceiling. The primed area of the ceiling is usually slightly lighter or darker than the rest of the ceiling, which may draw attention to the stained area.