There are two main rules to always remember about mirrors: Moisture is the enemy of any mirror, and Simple is best.
The very best and safest cleaner for any mirror is clean, warm water applied with a soft, lint-free cloth and dried with a clean, soft, lint-free cloth.
However, if that fails to work, a store-bought cleaner can work. There are few restrictions: Never use cleaners that contain ammonia, vinegar or any acid or alkali cleanser. All of these can attack the front surface, edges and backing of the mirror resulting in "black edge" or "housekeeper's mark" - a black corrosion that occurs when the metal layers have been corrupted with moisture or harsh chemicals.
Care should always be taken to avoid getting the edges and backing of the mirror wet. Cleaners should never be sprayed directly onto the face of a mirror. Always first spray any cleaner onto a soft, lint-free cloth first, and then wipe the mirror dry. Should the edges or joints get wet, they should be dried off immediately and completely. Never let moisture pool at the bottom edge or run down the face of a mirror to the edge. Always make sure there is proper ventilation in bathrooms where mirrors are installed to prevent mirrors from 'sweating' and having moisture condense and pool at the edge.
Never use abrasive cleaners on any mirror surface; they will scratch the glass.
To remove surface marks or stubborn dirt, use 0000 oil-free steel wool or a single-sided razor blade at an angle to gently scrape off any offending marks. (use like a painter removes paint from a window during clean-up) Take care not to gouge the mirror.
