Short answer
Hard water spots on a mirror are the same mineral deposits as on shower glass, and come off with a mild acid such as diluted white vinegar. The difference is the back: a mirror's reflective layer can be damaged by liquid seeping in at the edges. Spray the cloth, never the mirror, keep liquid away from the edges, and dry it completely.
Bathroom mirrors spot for the same reason as shower glass: toothpaste splashes, face washing, and water that dries on the surface. But a mirror needs a little more care, because the part that makes it a mirror is on the back.
Why mirror edges turn black
A mirror is a sheet of glass with a thin reflective metal layer, usually silver, on the back, protected by paint. If water or cleaning liquid keeps seeping in at the edges, it can attack that layer. That is the dark stain you see creeping in from the edges of old bathroom mirrors, often called black edge. It cannot be cleaned off, because it is behind the glass.
How to remove the spots safely
- Spray the cloth, not the mirror. This one habit protects the edges more than anything else.
- For chalky spots, dampen a microfibre cloth with equal parts white vinegar and water and hold it on the spots for a minute or two.
- Wipe with a clean, damp microfibre cloth, then buff dry with a dry one.
- Keep liquid away from the edges, and wipe the edges dry at the end.
Avoid abrasive powders and scourers on mirrors, and never let a cleaner run down and pool along the bottom edge, where it does the most damage.
Keeping mirrors clear
- Ventilate after showers. Run the fan or open the door so the room dries quickly.
- Wipe splashes as they happen, before they dry into spots.
- Seal the glass. Mirrors take the same clean-and-seal treatment as shower glass; as always, keep liquids off the edges.
For the full cleaning method on heavier build-up, see how to remove hard water stains from shower glass.
