Then I started to play with screengrabs in Max or directly on the textures in Photoshop, quickly adding details while trying to maintain the characteristic minimalist look. I got most of the ideas from military airplanes or tanks, and when I started to search for scale models decals, I found loads.
Any metal, plastic or rubber surfaces of a vehicle will have some common elements that will determine how old, worn and ruined that surface will look. Those elements are mud, dirt, dust, rust, scratches and so on. I took a sample object from my model that had pretty much all the shaders I needed, and I did all my experiments with that object, so I could play with the textures and shader values quickly, and then I replicated it to the rest of the vehicle for a final tuning.
I wanted to auto-generate a map for each of those, and then work to correct and improve them in Photoshop. So I made extensive use of the VrayDirt map. I created a V-Ray material, and in the diffuse I added a VrayDirt map. Playing with the values and adding different maps, I created and then baked four different maps:
1) Ambient occlusion (to be used as it was and as a mask for small dirt)
2) Convexity map (to give a subtle highlight to the edges of metal surfaces)
3) Scratches (very basic map to start with when painting scratches)
4) Mud (for the rubber of the tires)
I have to credit Neil Blevins and his SoulBurn scripts. There are so many useful tools there. Among them there is one I use often: texmapPreview. That script will render a preview of the map I have selected in my Material Editor. It's awesome when I need to tweak values (Fig.09).
Once I baked all those maps, I masked them in Photoshop and I removed all the areas I didn't want. Then I used the results as a mask itself for the scratched metal texture or the mud texture. After working on those layers, I started to add another level of hand-painted scratches, stains, oil leaks, etc. On top of all that I put the convexity map, masking out all the surfaces but the metal. The convexity map simulated the fresnel reflections. On top of everything I applied 50% ambient occlusion (Fig.10).
In Photoshop I try to be as least destructive as possible and I keep all the layers separate. In this case, when I finished my diffuse texture, I was able to create the specular, glossiness, bump and paint masks on the fly, just by adding masked adjustment layers (Fig.11).