... look for a filtration system that creates deionized water. You can buy the filters and rig up a system for your rinse water that will be just like what they use in car washes for the "Spot Free Rinse". It won't be terribly inexpensive, however.
Jerry
That will not work with hard water, as a matter of fact it will be worse.
Sixgun95, I respectfully disagree with you. I have a de-ionizing system that works very using the hard water in my town. Check out a company like CRSpotless.com (I have no affiliation whatsoever with this company, other than owning their product.)
As a test, I washed and rinsed a metallic black 300C SRT8 in direct sunlight using this system. After letting it air dry, there were no hard water spots at all. And yes, I was nervous the first time while waiting ...
The initial cost of the system and the consumable resin refills are not cheap, but works VERY well!
Mike