Having a tough time removing hard water mineral stains from toilets and sinks? Try a cleaning product that contains acid. Start with white vinegar, which contains acetic acid. Saturate paper towel with vinegar and place it on top of the stain. Wait a couple of hours and then wipe clean. For toilet bowls, pour vinegar into the bowl, wait a couple of hours, scrub and flush. If vinegar doesn't do the trick, try a cleaner that contains either hydrochloric acid (HCl) or muriatic acid, both of which are more potent acids. Follow the directions (and precautions) provided by the manufacturer. If done correctly, this should remove even the most stubborn stains.