1. The coating of the towel rack is not well protected, which causes the coating to be damaged. The brass and air contact, and it may grow "patina" in a humid environment.
2. It is not wiped often. Over time, some leeches form dirt on the surface, and eventually show speckled black spots.
3. Poor water quality, containing metal ions, coupled with periodic cleaning, or some hygienic dead corner areas. These metal ion-rich water droplets interact with the air, and may cause rust spots over time.