Getting hard water marks off a car

[3xpendable]

My mother in law (who lives in the US) has asked me how to get hard water marks off her Challenger. I believe the issue is she is washing it then the droplets are drying and leaving the 'teardrop' marks. She said no even glass cleaner is removing them from the windows.

So, what can I advise? I have already told her to wash the car next time out of the sunlight and then chamois it off.

[3xpendable]

Thanks guys, I'll let her know.

I'm heading over there at christmas so will give it a good clean myself