Make sure the water stain is dry, simply paint over it with a brush or roller and let it dry. Push the edges so that the paint blends with the surrounding area. What is a good primer? For interiors, KILZ and BIN are excellent stain blockers. When there is a water stain, remove any loose material, apply a patch and then apply the primer.
If the drywall and paint are solid, simply paint over the stain. In fact, BIN contains shellac; professional painters have used this stain remover for many years. If your home has recently suffered water damage, Williams Professional Painting will be happy to help you get everything back in order. Wash the surface with hot water and a liquid detergent solution to remove any contaminants, change the water frequently, and then rinse thoroughly with clean water to remove all residue before letting it dry.
Water stains can be very persistent, especially dark-colored ones that indicate that water has penetrated beyond the top finish and penetrates the wood itself. While mold is possible, water stains, caused by the reaction of water with paper, paint, and rust, can also form a black spot. If the water stains are on the roof, there is most likely a leak in the roof, probably near a vent, vent pipe, chimney, or some other element through which water tends to enter.