I used to park on this street where all the damn sprinklers sprayed onto the parked cars. What made it worse was that a lot of the tap water in L.A. is hardwater. Therefore, my car would sit there, take the water, bake in the sun, day after day after day. I knew it was bad for the paint to leave the stains on there but I didn't want to have to clean the car over and over again. So I finally moved away from there.
Now I needed to get rid of the stains since I didn't have to worry about future water stains. What to use:
Glass: 1/2 part vinegar, 1/2 part water; apply and wipe
Paint: Wash the car, use clay deep cleaner (damn I love this stuff), then wax and BUFF. You really need to buff to embed the wax into the car and smooth out the clearcoat since the water spots probably created microscopic holes in the outer layer.
This may not work if you've had the same water stains for years, but hope this helps anyway.
