Car washes have a major problem when it comes to hard water staining. The glass inside car wash tunnels gets hit by soapy water all day long, all week long. In conditions like this, there's almost nothing you can do to prevent the staining. It's just a matter of consistently maintaining the surface of the glass so it's not permanently damaged. I recommend getting the windows polished a minimum of 4 times/year. Ideally, you should get them polished 1/month to stay on top of the problem and maintain the window's clarity.