Wallpaper primer seals the surface to be papered and will allow the paper to stick better and be easier to remove. Since the surface is sealed, the water is not prematurely sucked out of the paste, nor does the paste soak into the surface, so the paper is easier to remove later. Paper can also be textured for painting. After you texture the wall, prime the texture, again with wall primer, before painting. Lee Roy asks.
Water trapped behind the mirror can cause the silver coating to turn black.
Chances are, a new mirror will be cheaper than resilvering the old one. If it is a large mirror, you could have the damaged part trimmed off and the edge repolished. Then rehang the mirror. Most mirrors that have this problem rest on the sink backsplash. When water gets on top of the splash, capillary action pulls the water up behind the mirror, damaging the silvering.