how do I prevent mildew/rusting in bathroom mirror?

Hi. In my bathrooom I have a mirror which was glued on the wall tiles using silicone. It pretty much fits on the wall (maybe 1-2 millimeters gap). Under the bottom , left and right sides of the mirror I see some gray substance. It probably is mildew or rust because of the humidity formed when bathing. Two questions: 1. how can i clean this ? (however, it is not much, so I wouldn't mind leaving it there) 2. how can i prevent it from spreading? I thought of sealing the 3 sides with silicone and leaving the top one unsealed so that any humidity trapped can escape. If humidity occupies space from bottom to up, then sealing the sides would be fine.
Your comments please
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On Thu, 15 May 2008 22:30:49 -0700 (PDT), interuser

If this is on the back side of the glass, the silvering is going bad on the mirror. It sometimes just happens but Windex will kill it very fast. If you must use a window cleaner, don't let it get to the bottom.
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check with your auto glass shop for replacement mirrors cut to size with various edges. see mirror trim kits. see hdsupplysolutions.
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Well, you have suggestions on how to get the cleanup done.
To stop it from happening, you need to do two things. 1. Open the door after a bath or shower, to let the concentrated humidity out of a small space. 2. Run a fan, so that the humidity moves away from the room.
The bathroom fan will not do the job, unless you have a very good vent and a very good fan.

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So any ideas as to why this is happening (only bottom side). Is it because moist fills space from bottom -> up ? More importantly, would filling with silicone the bottom part solve my problem ?
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You won't know until you try it. Good luck.
Joe
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wrote:

You won't know until you try it. Good luck.
Joe
--
no. most general purpose silicone is acid cure, and will hasten the mirror
rot.



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wrote:

Common silicones cure by reacting with moisture and release acetic acid. The (vinegar) acid is volatile and will probably dissipate quickly before any significant effect on the mirror coating. Worth a try, since the mirror protective layer may already be history.
Joe
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