creating dusty surface

[proundkontra]

Hi, i'm trying to get a nice dusty surface. some metal surface which layed somewhere for quite a while. and now there is a little dust layer on top of it.
I tryied it with vray blend material. Base material is just a metal shader. in the coat material shader i played arround with the roughness. bnow ut i just cant get a nice result.
does any one now how to approach a nice result?
thanks

[bazuka]

got some ref images?

[proundkontra]

hmm i couldnt find something that matches verry well. but i try to achieve something like that.

thanks