Hard water stains develop on a number of surfaces, including plastic. When hard water comes into contact with the plastic surface, it dries leaving a cloudy, unattractive film that makes the plastic look unclean. The easiest way to remove the hard water stains is to grab a few items from your cupboard, and begin the stain removal process.
Dip the edge of a cleaning cloth into a dish of pure white vinegar. Vinegar will dissolve the marks without damaging the plastic or leaving a residue when dry.
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Wipe over the hard water stains, using circular buffing motions to get rid of the marks.
Dip a toothbrush into the vinegar and scrub hard stains that are not removed with the cloth alone.
Soak the plastic overnight in a 50/50 mixture of water and white vinegar for extremely tough stains, if possible.
Scrub the plastic with a cloth the following morning, and allow it to air dry.