IF you want your mirror to really look like a glass-with-metal-backing mirror..... Try to do something close to that!
You can get some mighty nice "mirrors" by painting in the mirror with silver (almost all real mirrors were historically made with silver, and later with aluminum... so the silver color is pretty close to what the mirror color was in most cases) - and then add the "glass" by painting over that "metal" with some clear gloss or "Future". Future, which is a self-leveling acrylic floor finish, will form a very smooth glossy surface over your paint.
You can of course do exactly the same thing if you make your mirror with some metal foil glued down onto the mirror piece, and paint over it with some Future for the glass look.
Just a suggestion!
Bob