While superstitious people may be wary of a newly cracked mirror door, depending on the size of the damage, repairing the crack at home is a fairly simple procedure. Before taking steps to repair the crack determine if the damage is repairable. Small spider cracks and chips can be easily repaired with a glass repair kit, which you can find at your local auto parts store. Larger cracks may require a professional consultation.
Clean the cracked area as thoroughly as possible, using a soft cloth and glass cleaner to remove dirt and debris. Use a safety pin or straight pin to remove any leftover debris, then dry the area with a clean cloth.
Read the instructions that come with your glass repair kit. While do-it-yourself glass repair kits are offered by different companies, most include a stabilizer or base that features four suction cup "arms," adhesive, an applicator with plunger, a tube of resin and curing film. Apply the stabilizer over the crack by placing the round center section directly over the affected area. Press the four suction cups down around the crack to secure the stabilizer to the mirror.
Thread the applicator or injector into the center area of the stabilizer, and use your hands to screw the applicator into the stabilizer. Remove the applicator's plunger and fill the applicator with the tube of resin. Inject as much resin as needed until the crack is filled. Remove the resin tube and screw the plunger down into the applicator. The plunger will become harder to screw the further down you go and helps push the resin into the crack. Loosen it for a few seconds to allow air bubbles to escape, then screw it back down and allow the resin to cure for one minute. Unscrew the plunger and remove the applicator and stabilizer from the door.
Apply a small amount of resin to the top of the cracked area and place the curing film over it, as indicated by your kit's instructions. Use your hands to smooth any air bubbles that crop up when applying the curing film. After the resin cures for about a minute, remove the film. Fix any part of the repair that is uneven by applying a single-edge razorblade over the resin. If the blade is sharp enough, you should be able to remove oozing resin in one swipe.