There are both negative and positively charged dust particles floating around in the air at all times. These particles will be either attracted or repeled depending on how your car is charged. Like particles repel and opposites attract. But thats not all thats going on with dust. It also has some mass, albeit very small, so gravity is working against you too. The only way you are going to keep dust off your car is to enclose it inside a down draft clean room equipped with hepa filters. Even then you can get particles as small as 0.001mm.