Some times when it rains, depending on the weather conditions, rain can have a tds as low as 15 which is for all intent and purposes, pure water.
If you are then driving at 70 mph, wouldn't that be worse on the paintwork than just cleaning with pure water on your driveway or where ever?
And wouldn't the car and van manufacturers have researched that in terms of the 'water based paint'?
My van, which has been scrubbed weekly for nearly 6 years with pure water using a Vikan made for vehicles brush, still has paint work and no rust.