One thing that might not be directly on topic but still.
What I found extreamly hard is when you have mix of metal and color. Its a pain to get the material.txt file right so that the metal is reflective enough and the color isnt too reflective.
Next thing that is hard is to get the planelines right. If you have dark enough planelines for metal (on bare metal you do see em quite well) they are way way to visible on the color surface.
What I did was that I created a double set of planelines and rivets. One set above the color layer and one below it. I could have just created one set for colored areas and one for metal areas but since each plane is differently colored I opted for just duplicate layers.
Tex