Mr. Houdini-
Nice M6!
I think you may need to adjust your wash process, otherwise you will perpetually have swirls. Swirls or marring are usaully introduced during washing or drying with the incorrect technique.
What are you using to wash your car and what is your process. Black requires a little more attention to keep swirl free. Are you QD'ing between washes??? On a slightly dusty black car and QD wipe down could be a bad idea.
For polishing out the swirls try FPII first with a white polishing pad, then move up to IP if the FPII does not remove the swirls. The z5 has "some" filling ability, but it's not a swirl remover, they will be back.
Stick with zaino for the winter, the durability and looks are there for quite a while.
Here are some tips for applying zaino
http://www.google.com/ig?hl=en
Why are you driving an M6 in the winter? :nixweiss
Good luck-
Steve