Here is a question from Panda.
How can I clean dried on, very hard water from windows, both house and car ?
The professional auto detailer on our forum suggests that you use super fine steel wool and water to remove the stain itself, and then clean the window as you normally would with glass cleaner.
Hard water stains are removed using acidic cleaners as well. CLR® or products sold as soap scum or rust removers are typically acidic.
Here are a few home remedies to try:
Use pure white vinegar in a spray bottle to work on hard water stains on glass doors. Many people use vinegar to clean coffee makers
Cut a lemon in half, or use pure lemon juice to remove hard water stains.