When I first started collecting clocks, I did a bad thing. I bought a Ansonia short drop school house regulator. The label was in great shape, but coming off the back broad.
I asked my old clock repair instructor, Joe, what to do. He said that I should use white carpenters glue and dilute it with water to the constancy of a very thin paint, almost a light water color. Then paint the label till it stopped accepting the glue. I did this and it reattach the label and preserved it. So far, the label is still very clear and has not yellowed, its only been 16 years so far. I am not sure I would ever do it again though.