It's not hard, with a little practice to create a dirt layer. I use PSP. But other paint programs are the similar. I add a raster layer. then take brown or black and airbrush a small area like a corner of a fender. Erase unneeded parts, like the sharp edges. You can smudge, or push the small area to deform it like a splashed area Use different style brushes to get different effects . you can add noise. then by copping and pasting in new layers add the same dirt and road grime to other areas of the car. by adjusting the opacity of a layer you can lighten it or darken it to your taste. You can do a "bug" layer much the same way on front of the car. Also add some small streaks around the fuel filler that look like spilled fuel. If you look at the P3/P4 Ferrari skins they have some good dirty skins that could serve as an example.