Yes, in general a separate selection will allow a separate material assignment no matter what rendering plugin you’re using including the default Rhino Render. If you want to keep the solid as one, use a rendering plugin that has the ability to blend materials based on texture maps. In Brazil for instance you can use an ‘advanced blend material’ and a black and white texture to transition between two different materials. I often use the separate selection technique myself and Group rather than Join the objects for moving them around the scene.
I would like to see an option to apply material to faces/surfaces within an object without having to explode it, but I don’t think that is possible at the moment ?
Thanks @BrianJ! I am playing with Flamingo nXt. Possibly I can achieve what I want with several planar decals projected in different directions. Fortunately, the black paint is without texture.
What I did now is to create a 0.05 mm thick closed offset on top of the surfaces that are painted. Then I assigned a matte black material to that solid. This is of course a realistic representation of the paint. However, it introduces additional geometry taking up memory and resources, and tweaking the model becomes harder.
In 3ds max, there is an option called “Material ID” for different mesh facets groups - it is made exactly for the needs described here.
I wonder why there are still no feature such as Mesh Material ID for polysurfs in Rhinoceros…