I always suggest rendering in passes for later compositing
I prepared quick example of car paint for you (maybe a bit exaggerated one
FIRST - some car paint shader science:
this document years ago made me understand a bit what makes car paint:http://www.hello-napalm.com/tuts/brzl_shaders_04.html
look at the images in these pdfs:http://developer.amd.com/media/gpu_assets/ShaderX2_LayeredCarPaintShader.pdfhttp://www.mariomalagrino.com/car-paint-material.pdf
- we need several layers
1. Base paint with soft specular (in my example it has very different colours for ambient, diffuse and specular, but it's not necessary).
2. Fresnel (angle dependent) reflection of environment + sharp highlights (comming from coating layer).
3. Reflective/non reflective flakes layer (i.e. sparkling like glitter or mica) that typically occurs between these two layers - optional.
4. Other coatings (light/angle dependant) - optional.
MAKING OF (Anim8tor):
1. Base paint does not need no explaination I think.
2. Reflection material is made out of sharp specular without diffuse/ambient + specular reflection attribute for ART renderer. You will NEED some environment to be reflected in your paint. I've googled and quickly set up some random LDRI as an enviro map ( http://www.openfootage.net/?cat=6 http://openfootage.net/Openfootage/Vorschau/ContactSheetGaisberg2.jpg
), but you can use whatever you decide is suitable. You'll need fresnel mask as well (as I'm not sure if ART supports it directly), but that later.
3. For flakes layer I used the same as for reflection pass but made specular softer and wider, and changed attribute to glossy. Then I took advantage of low antialiasing settings (9 samples in this example) and lower resolution (to make flakes bigger: to have 4 px instead of 1 px - for speed and demonstration purposes). This is also good idea to play with some marble textures in this shader.
4. Masks are rendered with simple white diffuse (better with no specular or ambient) on white or black background - depends on need (without enviro map/other objects). Lighting map is simply our object with this material rendered on the black background. Fresnel map is rendered with single light located in the exact location of camera (turn other lights off and turn light located wel... kinda under the camera on) with the white background this time.
Read about masks more in Dotan8 - our Anim8tor zine from March 2010: http://www.dotan8.com/html/issues.html
(You've got also basic car paint example in that article)
MAKING OF (GIMP/Photoshop/whatever else that has the layers):
1. Base image
2. Reflection layer with inversed fresnel transparency mask
3. Flakes (2x of original layer size, scaled without interpolation, then I spread pixels a bit using filter to make the layer loose its pixelated nature) with light mask.
4. Other layers and combinations (for example I put fresnel to light up the edges and coloured it to the green shade). Then play with blending modes, colour balance, contrast and lightness of the layers and their masks.
If you go more into the topic, use more textures, masks, colours you'll be able to get some fantastic effects, maybe even some chameleon car paint type?
If you have some questions or problems with the setup just drop the comment in this topic and I'll reply ASAP. I'm not dead yet, I'm still visitor of these forums just didn't have a time to make 3d works lately.