Many mirrors require repair of some sort as time goes on. If you have a beautiful, intricately designed mirror, or an antique, simple replacement will not do. One option is resilvering the mirror which will, in most cases, restore the mirror to its original condition, or even better.
Determine if resilvering the mirror is your best option. Bathroom mirrors should usually be replaced as it will be less expensive and simpler. Many mirrors can be "backed" by a new 1/8-inch thick mirror cut to fit the original mirror. For more delicate or antique mirrors, resilvering may be the best option.
Clean the mirror thoroughly. Take care to notice any scratches or blemishes in the glass as resilvering will likely make them even more noticeable.
Don all of the safety gear mentioned above and use the provided chemicals to remove the original backing and silver. If your kit does not contain these things, a varnish remover should remove the backing. Nitric acid and water will remove the original silver. Mercury and other dangerous toxins will be present.
Clean the glass again using a soft cloth and the acidic cleanser that should come with the kit. It is imperative that you do not touch the glass with your hands from this point on.
The glass should be on a level surface as you apply the chemical mixture, which is the new silver of your mirror.
After about 10 minutes, the silver should be deposited evenly on the glass. Rinse with cold water and gently swab the surface with a cloth. Allow the mirror to completely dry.
Apply the new backing, usually a black lacquer paint that should be included in your kit. Once this dries, you may hang your newly restored mirror.