no offence to the zaino users, but replace zaino with opti coat, or the professionally installed opti guard.
the contaminants will come off easier and without clay.
Also with Opti coat, you could use ironx x instead of clay to wash the contaminants off instead of claying all the time (which will eventually lead to polishing).
also, ive read alot about zaino attracting dust and dirt...like an electrostatic charge that acts like a dirt magnet.
as for the swirls, proper wash techinique after they are removed. BMW clear is one of the hardest being a ceramic clear. id look to Menzerna PF 2500 for a one step, or FG 4000 for a heavier cut, but a polish like finish. they should take care of the swirls.
the problem with a dealership prep, even on a used car, is that they often use glazes...they are high shine products which contain fillers. the car looks great fora couple of weeks until the fillers wear off, then you see the real condition of the paint. the only way to remove the swirls is to abrade them out with polish or compound. then get a tough layer of protection on the car.