Car body 2
To start with for a basic material I used a fresnel falloff for reflections, set the IOR to 2.1 in this case, there is no magic number though, that would change in different environments, whatever I think seems best at the time, also on this material I used a large specular to simulate the metallic paint.
here I have added a couple of the decals from the cd, as well as added some of the areas of darker colour using a fill layer and painting on the layer mask to control where it is applied, I didn't expect too much from this but I think it came out quite nicely, its not to obvious and just gives it a bit more variation.
I have used the dirt maps that come on the cd, combining lots of them together to get the dirt in the places I want it, and using layer masks do erase from what was there. I started by doing smaller parts on separate layers and when I was happy, merging the layers together so I could paint though what was several layers in one go, to make streaks from water/rain, that kind of thing.
After this I also made a mask for the reflection to not reflect the dirty areas, as well as a mask so that the spec doesn't not effect the dirt OR the decals which would be stuck on top of the original paint work.