Water stains appear on walls for any number or reasons, including spills, window leaks, broken pipes, and condensation. Stains are ugly and often darken over time. It is important to remove them as quickly as possible. Simply painting over them is not always enough because water stains that have certain minerals in them sometimes bleed through paints and even some primers to appear again. Thankfully, removing a stain is quick to do and, after painting, will be unnoticeable.
Find the source of the stain, and stop the water from reaching the wall. This may be as simple as caulking around a window or as difficult as repairing a broken or leaking water line.
Dry the water stain completely. If the stain is still damp, be sure to wipe away any remaining water with a dry cloth, and dry it completely with a fan, heater or hair dryer. Allow the now dry wall to sit overnight, and verify that it stays dry during this time.
Pour a tablespoon of bleach onto a clean, non-abrasive sponge, and gently rub at the stain. Wear rubber gloves while working with the bleach, and scrub from the inside of the stain out. Rinse the wall area with a clean, wet sponge, and allow it to dry overnight.
Use a light-grit sandpaper to carefully sand the stain away from the wall. For drywall, it may be necessary to go back and cover the sanded area with a thin layer of drywall mud using a putty knife. Once the drywall mud is dry, sand it smooth to blend in with the rest of the wall.
Paint the stained section of wall with a stain-blocking primer to help ensure the stain doesn't bleed through the finished wall paint later. For best results, use a few coats, and let them dry thoroughly between each other. This will help ensure that the stain doesn't bleed through regular wall paint applied after the repair.
Test the use of the bleach on an inconspicuous segment of wall before attempting to use it to remove the stain. Not all walls and paint treatments react well to bleach.
Wear rubber gloves and a face mask when working with the bleach to protect your hands and avoid breathing the bleach fumes for a prolonged period. Make sure the cause of the stain is corrected prior to repair or it will likely reoccur.