Long time ago while I was still driving a metallic silver 1992 Honda Accord, I wasn't that particular in detailing. One fine day, I had my car washed and drove out for a spin. Unfortunately the weather turned bad while I was on my way back home and my car was soaking wet. Right after I reached home, I hose it down with pipe water hoping that the dirt will not stick on the surface before the rain water dries up. I did a big mistake by not drying up the car and let it air dried!
The next morning, I noticed that there are lots of white water mark rings on the paintwork. It was very hard, not even my nails could scratch it away. I then took out a bottle of Autoglym Paint Renovator and start rubbing the paint down. This aggresive polish contains aluminium oxide which is able to cut a fine layer of paintwork. And after 10 hours of hardwork, only I managed to remove all of the water marks.
This incident had taught me a lesson of not letting pipe water to dry on the paintwork. The reason being pipe water contains minerals and it will leave traces of elements as it dries up naturally. These elements will then etch into the surface (be it paintwork, plastic or glass) and leave ugly marks.
So, be wise! Spending 45 mins drying up the car or don't wash it at all! The choice is yours...