If a bike catches dust, then it means the owner have sprayed the bike or cleaned the bike the cloth which is oily. That's the major reason. I personally experienced. Once I serviced my bike in service station and after service the boy sprayed for shining looking, but after passing through the dust, my bike looked very awkward then I again cleaned my bike with water & SOAP then the problem solved. Now i just oil the necessary parts but not the full bike ..