Dirt on Car Paint

[Furnace]

Hi,
I have a simple red car paint, just a Vray material with a simple colour for diffuse and a Falloff for reflection.

The car is looking too new, does anyone have any ideas of how to add a little road dirt to the lower panels, I'm guessing a Vray blend but not sure if this would work, what would make a good light dirt, a texture or a procedural and what about the mapping on?

Many thanks for any advice.

[Mokiki]

VrayBlend is perfect for this. Use the Carpaint as base material and whatever your dirt is supposed to look like as coat. As for the layer mask I bet you can generate some pretty good black and white pictures from images found via google image search.
The easy way to apply dirt where you want it without the need for unwrapping you could use the "Poly Select" modifier and select roughly the faces where you want to add dirt. After that you use the "UVW Map" modifier with plane projection + specific mapping channel to place the mask exactly where you want it.

This can be easily applied to decals etc. too.
On a side note you don't have to do the poly select step if you only see the car from one side. Because if nothing is selected the plane projection will project through your car. Meaning you will have the same texture on both sides, depending on the orientation of the projection.

Hope that helps