When I detailed we used hull acid, from boat supply stores. For dark rides we diluted it a bit, but that was always followed with a buff, polish and wax. I've never used a clay bar on hard water spots, but I can see it working on the same principal as light overspray. I would also be really hsitant taking a powdered cleanser to the paint of anything that you want to be shiny. It would probably work well for the windows, but I've had great luck with .00 steel wool and vinegar.
LOL... this brings back memories... I remember one guy spilling a jug of wheel acid and bleaching this huge spot that looked like a penis into the concrete in front of polished image... the old owner was so pissed, I thought he was going to explode.
