Not sure I understand, but CLR removes hard water stains
If you are talking about the paint on the exterior of the car, you would be best off by using something made for cars so that you don't damage the clear coat layers which is protecting the colored paint underneath and help give the paint job that look of depth for the paints they use today. Otherwise, try using water and dry it immediately. If you know of someone who works in a body shop, they would be able to give you the name of a good product to use.
I have listed some videos for you that address this very question.
Use Clay bar and then wax
Hello Juan, this video may help you