Unless it is something really special, you should replace it. It is
possible to repair such a mirror, but it involves removing the reflective
coating on the back and replacing it. Unless it is something really special
that will be more expensive than just replacing it. You can not just clean
If this isn't a special/ antique mirror and you simply want to repair it
because the blotches are unattractive, here's another possible solution for
If the blotches are in an unobstrusive area, e.g. in one corner or near the
edges, you might try using an etching spray. This is available in spray
cans, and as the name suggests, gives the mirror (or glass) an etched look.
It can be applied over the blotches, either in a pattern with a stencil, or
by taping off the desired section. Obviously this won't really fix your
mirror and will cover the portions that you do spray, thereby making those
unusable. But this technique might just make your mirror look nice enough to
where you would be willing to keep it. I have no idea whether this is an
option for you, but I thought I'd throw it out there.
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