Hmm this is a post that would be best ansewered in the render forum but here goes.
Make two materials of your liking, surface shader node and a blender node. Make the other material the metal that shines thru and the other a painted version.
Connect them both to blender inputs... make a alpha map and connect it to the blender chooser then connect this to the surface shader...
ormake the shiny one transparent on parts (remeber to kill specular in the same manner)and layer it on top of the painted layer (or other way around whatever suits your taste). But i suggest using the former method.
