I decided fairly early on that the car should be muddy and dirty. This mud would need to be textured onto the car from bitmaps, which would mean huge file sizes if I wanted to render close-ups. I needed a system to allow me to tile and reuse my mud textures so they could stand up to extreme close ups.
Painting textures into simple Photoshop files and just applying them to the car wasn't going to work.
The solution I settled on was a fairly simple one, using masks to define where the mud would lie on the car, and then mixing through to a mud shader in those areas. The mud shader was mapped separately and could be tiled and varied without affecting the paintwork.
I separated the dirt into two layers a dark wet mud and a thinner dry dust spray, each layer had its own mask for each body panel. The mud itself was mapped globally to the whole car with a tri-planer projection map. This way the mud would run from one panel to another neatly.
Mostly importantly this approach meant that I could deal with the look of the mud separately from its placement.
Once I'd UV mapped each body panel I was really to paint my dirt masks. I used Maya's 3D paint system to paint myself guides on each panel, and then exported these TGA's out to Photoshop and painted more finished versions.
I also exported the UV coordinates at the same scale and put them into the Photoshop file, this gave me exact reference of where my paint strokes would appear on the car in Maya.
I then worked on two masks for each panel, a basic dry dirt mask and a wet splatter mask:
The black areas define where the mud will sit and the white where the paint will be.
All the masks were hand painted and assembled from various sources. I used a lot of concrete texture maps layered up and manipulated to create the basic dirt masks.
The paint shader
The red paint shader was fairly simple, although given the strange way mental ray's FG system seems to deal with shaders it took a lot of tweaking to look right. All the lighting in the scene came from a HDRI map.
The shader is a simple red Blinn with the diffuse component turned right down to 0.4. The reflectivity and specular colour have a sampler info node passing through a curve to really push the specularity when the metal bends away from camera.
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