On several walls in my home it looks as if someone threw water on them and let it run down the wall. The stains are underneath windows mainly and one is on the other side of the bathroom. What causes this?

 

Likely steam from bath condensing on the cold walls, moisture from air condensing on the cold windows collects and runs down the walls.

 

I'd agree with chfite, as to the cause, bout the only thing I've found to hide water stains on drywall is kilz or zap. There maybe other products sold in different areas, but you're probablly gonna have to use an oil based primer to cover it up.

 

As indicated, you have a condensation problem. This is a result of warm, humid air condensing on a cool surface. Reducing humidity level in the home with a dehumidifier will help. Running and adequate sized vent fan in bathroom during bathing and at least 20-30 minutes afterwards will also help. Check your windows and make sure they are properly fitted and sealed and not leaking cold air inside. If you feel cold air leaking in, then you are also loosing that much heat to the outside.

If an all purpose cleaner will not remove the stains, then as advised, you will need to seal and prime before repainting. Doing so, will be futile if you do not address the source of the problem.

 

Twelvepole, you are of course completely right about finding & fixing the exact cause of the problem, prior to treating the symptoms. If he doesn't fix what is causing the problem, covering the stains would only be a temp. fix at best.