Hi!
Does Vray Dirt work differently in Maya and Max? In Max you can add a textured bitmap to the Radius and still control it by a number, to simulate for example mud. Not possible in Maya, or is there a workaround?
I remember seeing somewhere Dirt being used as a snow or dust effect (ONLY affecting top planes of objects). I’ve tried ‘Invert normals’ and changing Bias Y, but not there yet. Any advice how to create that snow/powder/dust cover effect?
I believe still the best method is the snow shader in Maya, have you tested with that? you can also use the snow shader as mask for a Blend material so that the dust layer can be achieved over lets say a shiny surface.
You can try the Vray invert normals option but is not quite the same effect…the invert normals is good for darkening the edges of a surface like old wooden boxes for example.
Gooseneck: I actually just tried that. Two materials controlled with a Maya Snow texture in a Blend material. It did the trick! Thanx 
As for getting VRay Dirt to only affect top surfaces, mask it with VRayFalloff towards/away from world Y.
dgruwier: Thats interesting too. Seems like a good alternative to SamplerInfo/Fresnel. And a Mix curve to control the amount. Cool, thanx.