Claying Windshield

Is it safe to clay the 300's window if it has the auto wipers?

I've got some minor hard water spots on my windows and have read that clay can be used on them. There is even a kit (by Diamondite - not sure of the spelling) for cleaning windows that appears to come with a small pad of clay.

The owner's manual advises agains using stuff like Rain-X since it can affect the sensors for the auto wipers. I'm not sure if there is something on the surface of the glass that the clay will mess up.

Thanks for any help,
RP