I use Dawn dishwash. detergent everytime I wash my car...it helps strip the car of any water spots or deep grime and most importantly...of old wax so that it doesn't build up. Then I take a clay bar to the bumpers and lower half of the sides and remove tar buildup etc.THEN i spend about two or so hours buffing out the paint with a three or four step process lol. If you want a quick fix then wash the car with water and a little amonnia...shock the **** out of those hard water spots but shouldn't ever harm the wax OR the paint on the car itself. To be on the safe side though i would go over the whole car with a quick detailing spray that costs maybe $5.00 at an auto parts store to bring back the gloss in the paint.
Good Luck...and be careful...automotive paint can be VERY tempermental to work with at times!