Hard water stains are caused when water with dissolved minerals is allowed to dry on glass before it drips off. This is typically caused by sprinklers regularly spraying against windows, or using a hose to spray off windows and railing glass. As mineral deposits become thicker and more encrusted the rate of accumulation increases because the water droplets have something to stick to and don't roll down the glass.
We use a combination of specialized cleaner and soft pad to dislodge the minerals from the glass surface. Depending on how long the minerals have been accumulating it may take us more effort and repeated treatments to get the stains to release from the glass surface.
The best way to prevent hard water stains is to prevent hard water for hitting the glass to start with. This means adjusting sprinklers heads, or installing shields to prevent overspray from blowing and landing on windows. A whole home water softener can be a good option for people that need to hose off decks regularly, although softened water may not be practical for irrigation systems.