No expertise at all, but just to clarify for others: This happens after you wash the car yourself, right?
One suggestion: I know with the Meguiar's wash stuff I use, it's recommended that one not spray at all, but simply run an ordinary non-nozzle stream from the hose to rinse, as the gentle flow works with the chemistry of the wash solution to enhance a sheeting action and minimize water spots. Try losing the nozzle for a wash and see if not spraying water at all, but simply running it over the car as needed, makes any difference.

__________________
Craig Bethune

'97 SL500, 40th anniversary edition

'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)


SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)