why isn't there a system for car valeters?
There are many valeters/detailers who use Pure Water through a DI Vessel for rinsing-off.
You can use tap water with shampoo and then rinse with tap water.
For the final rinse, use the Pure Water.
If your vehicle has a decent coat of wax, use a slow open flow of pure water to 'chase' the water off the paintwork, this method will leave the vehicle almost dry, and what little water is left will leave no watermarks/spotting.