I have a mirror to frame & mount in the garden. What's the best way to weatherproof the rear of the mirror to protect it from moisture?

WRONG FORUM. This is the Cars & Transportation Maintenance & Repair forum. Please move your question to the  Maintenance & Repair forum in the HOME & GARDEN category. Thank you.  

Epoxy enamel should do the trick.  The most vulnerable area is the edge:  I think I might put something like flexseal tape on the edges, then paint over that.  But it may not be enough:  perhaps you have seen mirrors in your bath that are affected by water drops on the room side:  the water has not penetrated but apparently just the temperature differential is enough to disturb the rear side coating.  

Are there purpose-made out door mirrors?  How about the mirrored glass used on buildings?  Our condo building has those:  exposed to 40 years of Atlantic coast weather and still OK - - - the reflective coating is on the outside of the glass.  And I know that companies such as 3M make reflective window film to apply to the inside of windows for sun control.

Tar & Tar paper.