Shut off the stove. Let the glass surface thoroughly cool.
Drench a white paper towel in white vinegar. Lay the saturated paper towel on the stove top, covering the hard-water stain.
Leave the paper towel on the stove top for an hour. Vinegar dissolves mineral deposits on stove surfaces.
Peel the paper towel off the stove top. Wipe the stove with a dry paper towel.
Inspect the stove top for remaining hard-water stains. If any white film remains, douse the mineral deposits with ceramic cooktop cleaner.
Cover the moistened mineral deposits with a water-moistened paper towel. Leave the stove surface alone for 30 minutes.
Remove the moistened paper towel. Scrub off the remaining water stains with a damp sponge. Rinse the sponge and wipe the stove to remove any cleaner residue.
Dry the flat-top stove thoroughly with a dish towel.