Not the 1978 film directed by Werner von Fassbender, but the stuff that accumulates on the car.
Even if you leave a car in a nice dry underground carpark, dust eventually settles on a car’s bodywork. In urban areas the dust is a mixture of exhaust particulates, pollen and vegetable matter such as pollen and leaf fragments. We breathe this stuff in all the time.
During summer when the humidity is on the low side, this dust usually remains mobile. As winter settles in and air temperatures drop, the air moisture tends to
condense around anything small and pointy. This is something to do with electrical charge, I am told. Upshooting from this fact, the loose dust on your stored car now darkens and begins to adhere to the bodywork as an oily, sticky film. A general dullness drapes itself over the car. I notice that this film builds up not only on the horizontal surfaces but the near-vertical ones as well. Again, I think electrical charges cause the dust to settle sideways. The damp only adds then to this stickiness.
The process I described above takes several months and needs a still, dry space. Vehicles left outside will also gather a film of dirt of another kind. Have you noticed this is seldom as even and homogenous as the dirt that accumulates inside? It seems to be be coarser and more speckled. Sometimes it is black and sometimes it can have a greenish cast. This kind of dirt also takes some considerable time to accumulate.
What sort of dirt is it that car manufacturers are worried about when they ship cars? I have noticed that cars in transit to dealers are coated with a protective white film. It is not hard to imagine that the cutting paths used to define the sheets of film are by-product of the CAD data used to make the cars’s tooling. Are these white sheets of plastic film needed to stop dirt or dents or both? A few simple calculations would show how many hundreds of square kilometres of this plastic film are used every year on the short journey from the manufactory to the dealer. Is it ever recycled? And so we come full circle as we contemplate a white plastic, and doubtless rather polluting, film being used to protect cars which are coated in a film of paint, the job of which is to protect the car. From dust you can wipe off with a damp cloth.