We have hard water here too, and I just finished the black paint on my 1970. Water spots galore on all my cars until I started doing this.
There are a few things you can do, but the bottom line is to do your final rinse with de-mineralized water. From my experience, the spots dissolve almost immediately in water.
The least expensive way is to buy several gallons of distilled water from the market, and pour it over your car when your finished washing.
Second option, buy a Hudson Sprayer style garden sprayer. Fill it up with the gallon jugs of distilled water and final rinse the car with that. You know, the kind with a plastic tank that you pump up and spray weeds and such with. Use it only for rinseing your car, so it won't get contaminated.
If you don't want to keep lugging home gallon jugs, have a home-use reverse-osmosis system installed in your house or install it yourself. The added benefit is great drinking/cooking water etc. You can buy a good one from Costco.
This last idea is initially the costliest, but one of the best. Hook one of these up to your house/garage.
http://www.costco.com/Browse/Product.asp...&lang=en-USThey say if you plumb the water from your Reverse Osmosis water system into one of these systems, it will last 25 times as long. You will probably need to add a second larger pressure tank to feed it effectively though.
