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In an upstairs bathroom, I found these water trail stains on a wall.

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There's no plumbing behind the wall so I imagine these are from humidity.

What I'm curious about is whether there's any way to prevent such stains from occurring. Is there a special type of paint, or a water-resistant drywall, that can prevent this?

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    Ventilate ventilate ventilate – Kris Sep 24 '19 at 19:57
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    Not a special paint, but better paint in a satin or glossier sheen will make the wall easier to clean and wipe down. I agree with the humidity issue, but good paint will be easy to clean. – JPhi1618 Sep 24 '19 at 21:14
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As Kris said in a comment, the solution is to not let that much moisture accumulate. Run a well-performing bath fan every time you use the shower, even in the dead of winter. Vary run duration accordingly. In my part of the world this means 2-4 hours in summer and at least a quarter of an hour (after the shower ends) in winter.

As I often say, your bathroom isn't a bathtub. It shouldn't be expected to hold water, whether on the floor or the walls.

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A gloss paint would minimize this type of staining, but will do nothing for other issues humidity can cause (ie mold and rot). As others have said, ventilate! In the winter, simply leaving the bathroom door open is enough for my house (but then, I don't have kids, so the shower isn't running for more than an hour every day, and the bathroom is empty for long periods of time - ymmv).

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Ventilation is key to preventing moisture from building up in a bathroom. Either the vent fan may not be running long enough, or it is moving an inadequate amount of air. The vent may be blocked, or the fan itself is not working properly, or does not move enough air (CFM) for the size of the room. If so, then the fan would need to be replaced. If it simply is not running long enough, the fan could be tied into the lightswitch so it automatically turns on when the light is on. You could also install a timer on the fan so it will run for a longer period of time. If there is a window in the bathroom, opening it can also help drying things out.

The paint in the bathroom could also be a problem. If you are getting stains that won't wash off, you most likely have a mold problem. The surface of the wall will need to be cleaned off with something that will kill the mold spores such as a cleaner with bleach, or a product specifically designed to kill mold. I would then suggest priming the walls and ceiling with a latex primer that is rated for interior and exterior use (low VOC). If you want to be extra vigilant, you can also use a mold killing primer. For the topcoat, a paint that is specifically made for kitchen and bath is recommended. I also would recommend using a satin or eggshell finish that can be cleaned with a sponge.

There is special type of drywall that is mold and moisture resistant. Obviously, that would require completely replacing the existing drywall, which is not economical. It is not necessary to install that type of drywall unless it comes in direct contact with water, such as in a shower stall, or as a tile backer. It used to be common practice to use water resistant drywall behind tile, but nowadays most people use a cement backer board instead.

Repainting the room is most likely going to fix the problem with the stains. If moisture still accumulates on the walls, then at least it won't cause mold issues.

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I had this same issue in my kids' bathroom. Being teens, they often forget or neglect to turn on the bath vent. I solved the issue by buying and installing thin plexiglass and covering the drywall areas above the tub/shower surround.I caulked the edges and seams.

This works well and only cost me about $20-30 and an hour of effort.

This also works during power outages.

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