The main problem with using a cleaner like this is, while it removes all of the dirt and debris from the paint, it also removes any protection the paint might have on it, leaving it exposed to the elements and to UV rays which can cause faiding, oxidization, cracking, and pealing. Remember, glass is very resilliant stuff. It can withstand harsh chemicals (if needed). Glass cleaners in general will remove pretty much everything off of the surface of the glass, with any excess evaporating leaving the surface pristine. Your paint is not nearly as tough. Yes, it will clean it and leave it sparkling, but you are also taking any kind of protection it might have away. This will leave it open to the elements which will make it show its age much more quickly. You will do you car finish a much larger favor by cleaning it via normal means, then throwing some carnuba wax on top of it to provide it protection. This way the sparkle will not go away anytime soon.