Smoke or Dust Stains On Walls
Some stains just happen - you didn't drop or spill anything but there it is. Maybe you decided to move furniture around which involves hanging pictures in a different location. You take one down and find the wall around it is so stained that you can see where the picture was hanging. Atmospheric dirt and dust is constantly around us - indoors and out. This atmospheric contamination, which may also contain smoke residue if you have cigarette smokers in the house or use a wood stove, clings to walls and works its way in. It eventually sets and noticeably alters the colors of your walls. Before you resort to repainting the room, try washing the walls first.

1 Cup Household Ammonia
1/2 Cup Liquid Colorless Detergent
3 Gallons of Warm Water
Don't forget to protect your hands with rubber gloves as the ammonia, which is great for cutting grease, can be pretty hard on bare skin. Use a large household sponge and begin at the bottom of the wall and work up. Do section by section. Depending on the size of the room, this can be a big chore for one person. Enlist the help of your spouse, roommate, or teenager. Once you get started, it goes pretty quickly. The benefit is your walls are clean and you don't have to put up with the painting mess and expense. If any of the cleaning solution drops on carpeting or furniture, it will not leave a bleach stain.

Generally, water stains are caused by dust adhering to moist surface. If this is the case, washing the entire wall should do much to alleviate the stain.