There is an automated car wash round here that says it gives a final rinse with purified water. The BMW local dealer washes their cars through a purified water sprinkler system.
Washing with purified water cannot harm the car anymore than letting the rain fall on it. Of course it will reduce the wax finish on your vehicle, as soon as you finish waxing your vehicle the elements start to break it down!
If you love your heap of metal on wheels so much, you can wash it then wax it. The only difference using purified water, is that you can walk away, let it dry, wax it. Using tap water, you'll wash, rinse, dry it yourself, then wax. End result = Same.
If like me your van is just a work horse, then you just wash it down with wfp, 10 mins. walk away! Send it to the garage for regular service, thats it.
In short: Purified water does not speed up the rusting process, nor does it slow it down, it just saves time on the need to dry.
Oh and by the way, unless you give the windows you clean a wax finish, neither wfp or trad will make the glass shinier than clean glass can be. I still use both methods, in fact I use a third method too; ie. wet microfibre wash + dry buff microfibre. Glass looks exactly the same when done properly, however it's done.
