Q: Is it necessary to finish drywall before applying anything decorative to it? Also, what causes mirrors to turn dark around the edges after a while? -- Shirley
A: First, the drywall. The decorative finish which you select to install will have everything to do with how you will need to treat your drywall once it has been hung in place. However, regardless of the finish, you must apply joint tape and joint tape compound at every drywall joint if the drywall is going to be installed onto a wall that is adjacent to your garage.
Nail head indentations must also be filled with the same compound at this location. The garage-to-house connection must be separated with a wall configured to resist fire. The taped joints and filled nail head indentations are a part of a fire-rated wall configuration. Exterior walls should also be given the same finish treatment. At exterior walls taped joints help to reduce air infiltration -- a real energy-saver.
If paneling or built-in cabinets will be installed over the drywall a fancy finish will not be required.
Wallpaper gets treated a bit differently. It should be placed over drywall only after all drywall installation imperfections (joints, nail-heads, tears, etc.) have been smoothed with several thin layers of joint compound (sand the joint compound between each application with 80 grit sandpaper). Don't forget to apply a coat of oil-base primer before installing the wallpaper. And, remember, most wallpaper sizing applied to an unpainted drywall wall is absorbed by the drywall and joint compound, and is rendered useless.
If the decoration desired is a smooth wall of paint, two additional layers of joint compound should be applied. This is because the paint will show imperfections quite a bit more readily than wallpaper. We don't recommend stipple paint (the kind of paint the dries to look like an orange peel with all those tiny bumps). Making repairs to a stippled surface is nearly impossible. A textured (skip troweled) finish is recommended for most surfaces that will be decorated with paint.
The dark edge surrounding your mirror is a common problem. What's happening is that the silvering (the reflective surface applied to the back surface of the mirror) is oxidizing and turning gray or gray-black in color.
When mirrors are manufactured, a special coating is applied to the back and edges to prevent oxidation of the reflective layer. Unfortunately, mirror manufacturers make much larger pieces of mirror than we use in our homes. When the mirror is finally cut-to-fit by the mirror installer, the factory-installed protective coating is disturbed at the location of the cut (the edge), air gets to the coating, the silver turns black, and the mirror starts to look terrible.
A seal is sprayed on locally by some mirror companies, but may still not be enough to prevent the condition.
If the mirror in question is an antique it might be worth re-silvering. If not, replacing it would probably be more cost-effective. When the new mirror is installed, use paintable silicone caulk to seal the mirror to the wall. Doing so will keep moisture out, slow down the oxidation process, and the mirror will stay beautiful longer.