A small crack in a mirror will cause most people to dispose of a nearly perfect mirror when in reality they could easily repair the crack. Use a basic glass repair kit to fix a crack in the glass surface and extend the mirror's life. Glass repair kits are available at auto supply stores; the kit includes resin, film and stabilizers.
Clean the crack in the mirror with a soft cloth. Remove all dirt or debris from the crack and surrounding area.
Apply the stabilizer from the repair kit over the crack by peeling off the adhesive backing. This will cover the damage and allow you to repair the crack. Press any air trapped under the stabilizer with your hands so it is smooth against the mirror.
Fill the syringe with resin and insert the syringe through the stabilizer so it is in the crack. Inject the resin into the crack from the syringe until the crack is filled. Let the resin cure for as long as your product indicates and peel off the stabilizer.
Apply a tiny amount of resin on top of the cured, filled crack. Cover this resin immediately with the curing film.
Peel the film off after the top layer of resin has cured as well.
Smooth the repair with a razor blade if needed and clean the mirror with glass cleaner and a microfiber cloth.