Originally looking for the long poles/brushes to clean windows I discovered the "pure water" systems which many of the commercial window cleaners use and, especially as we are in a very hard water area (near Safrane), got one of these
http://www.windowcleancentre.co.uk/cleaning-products/Pure-Water-Kits/HydroPower-Di12-Kit.aspx [our house is fairly tall and I couldn't justify the cost of carbon fibre poles, so went for aluminium - my puny frame can only just handle them on full 9m] - anyway, today I decided to try the water for washing the [Black/NC] GS.
Amazing! I used the brush head with the water coming out of it for the car windows but didn't touch the paintwork, just watered that a fair bit from the brush head before using a microfibre mitt with the same deionised water in a bucket with a detergent / wax mix, then washing the whole lot off with the same deionised water sprayed from the brush head and wondered about drying it but decided against - no marks at all.
I'm certainly not a detailer but was very pleased with the lack of water marks and if you can do the man-maths to save on window cleaning and, incidentally provide <1ppm water for the Maserati....