Ceiling water stains are almost always caused by a leak. Whether you're noticing the stains for the first time or they're old ones, find the leak or you could soon have some major ceiling repairs on your hands. Painting over the stain isn't difficult, but it requires an undercoat of stain-blocking primer to prevent the stain from reappearing. Whenever you paint the ceiling, you must protect your eyes, the furniture and the floor.
Stop the Leak
By the time water stains show through the paint on your ceiling, the water has soaked through the gypsum drywall, and that means either a fast leak or slow one that has been going on for a long time. Either way, the first order of business is to find the leak and stop it. If there's a bathroom above the stained part of the ceiling, the culprit could be leaking pipes. If the leak is near a wall or on the top floor, there's a good chance that water is leaking through the siding or the roofing. The detective work may take effort, but it's effort well spent -- it could save you costly repairs in the near future.
Check the Drywall
Once you're sure the leak has stopped, you should test the drywall around the stain to make sure it isn't waterlogged or crumbly from deterioration. Set up a ladder tall enough for you to comfortably reach the stain from the second-highest rung and push against the drywall with your finger. If it feels wet, let it dry for a week or two before painting it. If the drywall feels spongy, and you can easily make a dent with your finger, it's best to cut out the damaged part and replace it with new drywall.
Apply a Stain-Blocking Primer
After you've determined that the drywall is sound enough to repaint, move the furniture into another room. Cover the floor with plastic and find a good pair of goggles -- drips are a fact of life when painting the ceiling. You need to cover the stains with primer before painting, and drywall primer won't do. Use shellac-based, stain-blocking primer or an equivalent high-solids latex primer. If your ceiling has a texture, you'll find using an aerosol can easier than a paintbrush, and you'll get more reliable coverage. It's just as easy and effective to brush a flat ceiling as it is to spray one, though.
Touch-Up or Repaint
You might get lucky and discover that the primer matches the ceiling color, but that's a little like winning the lottery. In most cases, you'll need to touch up the spots with the ceiling color, but keep in mind that the color of fresh paint is a little different from paint that has had time to fade and collect dirt. Touching up the spots with a paintbrush and roller is easy, and it's worth a try. If you want an exact match, though, prepare yourself for the possibility of having to repaint the entire ceiling.