If the black spots are on the "far side" of the mirror, not on the surface, the silvering is likely damaged and there's not much you can do about that aside from taking it down and getting it redone.
Use stickers on the "good side" to cover up the black spots.(01/15/2005)
Sounds like the back of the mirror got wet or cleaner behind it which will in time remove the coating on the back of a mirror. If it's along the edges, you could put some kind of boarder or frame around it. I have an old round mirror that was all black around the edges and I used the glue gun and some silk flowers to cover it up. Or you can purchase etching cream from your local craft store ($9.00 for 8oz bottle) along with contact paper (roll of shelf liner) and Exacto Knife and make a design on the mirror to disguise any blemishes. I have made decorative mirrors using the etching cream which have come out beautiful. Apply the cream liberally with paint brush and leave on for 10-20 minutes. Directions are on bottle. (01/24/2005)
The silver and copper layers are oxidized, you may buy new mirror with backboard in the future, then you can remove it easily when you meet this situation again. (01/07/2008)
By Andy Yu
Most black spots is caused by oxidation of the underneath coatings caypused by spraying glass cleaner on the mirror.
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