Hard water stains, which are actually mineral deposits, won't just wipe off your mirrors. They'll stubbornly cling to the surface until you find something to break them loose. Acid removes hard water stains, yet strong acids can etch the mirror. Vinegar is a safe, weak acid, capable of removing mineral deposits while not harming your mirror. When cleaning your mirrors, don't spray water or cleaning liquid directly onto the mirror. If the moisture from the spray gets into the edge of the mirror, and works its way behind the mirror, it will damage the mirror's silver.
Things You'll Need
Combine in a bucket equal parts white distilled vinegar and water.
Dip a lint free rag into the solution, and wring out the excess liquid.
Wipe the mirror with the vinegar solution, to remove the hard water stains. Let the vinegar solution sit on the mirror for a few seconds before wiping off. Repeat if necessary.