What I do is use Griot's Garage Speed Shine after drying. This removes the fine water spots nicely. I to have a black Z and the trick is to not wash in a circular motion, this will show swirls marks. On the top surfaces wash front to back, on the side surfaces top to bottom. I use Griots boars hair brush and sheepskin wash mit, kinda expensive but they have reduced the swirl marks. Most dealers will wash your car after services I highly recommend you check out their work, I will not let them wash mine ever. They have caused me untold hours of enjoyment buffing my Z. I also use the Absorber to dry and a Water Blade but some have had problems with these.