You're not imagining things; those drip mark lines on your bathroom walls do exist. When condensation from a hot shower or bath forms on the walls and begins streaking or dripping down the wall, drip marks occur. These lines result after the water evaporates. Mineral deposits remain on the wall and build up over time without proper cleaning maintenance. The best way to avoid these lines requires proper exhaust ventilation in your bathroom. The moisture in the air must be removed by some mechanical ventilation method. Excessive moisture leads to mildew growth.
Pour 1 cup household ammonia, 1/2 cup vinegar and 1/4 cup baking soda into 1 gallon of water. Mix these together in a mop bucket.
Fill a spray bottle with the solution.
Spray the walls with the solution and allow it to soak for 15 minutes.
Scrub the wall with a nonabrasive scrub pad, working from the bottom of the wall to the top.
Wipe the wall clean with a wet dish rag. Wipe from top to bottom. Repeat if necessary.
Open a window and door to provide adequate cross ventilation Avoid getting ammonia or vinegar in your eyes. Don't inhale ammonia vapours.