it's generally caused by acids, most likely cleaning products.
you can get a spray can of mirror sealant at stained glass stores, or you can use krylon spray lacquer, which is pretty much the same thing. spray it on the back edges. if it's under metal edges, you'll have to get the glass out before spraying.
We've had several bathroom mirrors that start losing the reflective material around the edge. It looks awful before long and has to be replaced, money we cannot afford. Is there someway to seal the edges? Is this caused by steam?
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