I talked at length with my glass & mirror man here and he told me to quit worrying. All mirror will eventually lose its silver. He said no matter what you seal it with or use to adhere it this will happen. Usually many years from now. So I just go on with my doing and use it as I get it.
Maybe someone else knows more, but my experience is not having any trouble so far.

Thanks Flag. It is a big issue when soldering glass and using flux. Just got to wondering how caustic mortar is. I like your approach and think I will just proceed. Don't know why I have this impulse to want things to last a hundred years!

I too want things to last a 100yrs!!! I spray the back of mine with a sealer coat...or I have also painted the backs with nail polish, but I have such big projects now I have just slapped er on! But def I used nail polish when I do stained glass/soldering!!!

I have had some that has beeen in the garden for years and still look good .I do not do any thing to the back.

Thanks for the answers. Just what I needed were others experience. I think I will use what I have cut but since I already have a can of mirroe sealer I will probably spray the mirror before cutting more.

It is my opinion, to seal the mirror After it is cut. I plan to use mirror on my throne chair, and will spray seal the mirror pcs...as I want that baby to last 100yrs!!!!heh heh!

O.K. got to agree, especially since I have the seaaler sitting on the shelf.

I have a question about the sealer. do you seal the edges only or the entire back? I've never used mirror in my stained glass pieces for fear that it would its silver.

I was taught to seal all surfaces except front of mirror. Using spray sealer I probably don't always get the edges as good as I should. The force of the spray and can move small pieces around and then you get some on the mirror.

At the SG studio I go to, they recommend black spray paint for mirror; also on the back of tile for mosaics.