Having washed my vehicles over many years, in various municipalities and on various wells, the only way that I ever found to prevent spotting is by immediately towel drying. It's a pain, so I only have towel dried prior waxing (and I haven't even performed that one for a couple of years). Now, it may lift the solids off the vehicle prior drying OR it may be simply spreading them so thin that they are unobservable. Either way, it appears clean.
Now, I have noticed that since I have switched to detergents formulated for auto bodies, the spots have greatly diminished, but not totally disappeared. (Currently using Armor All Car Wash Concentrate, but I'm not brand partial. Just pick up what ever is on sale at the time. Says on the bottle "Brilliant Spot-Free Shine'. Goes with the next thought…)
Von, I like your observation about the "SPOT FREE CAR WASH'. Maybe they are being honest, because they don't charge for any spots they leave. /w3tcompact/icons/smile.gif.