If you do the render clouds thing and use it in different channels, but the bump channel first, of your shaders (if you can do stuff with the 3D model) it might get better. If you use different clouds the resulting randomness makes it more real world-like. I once made metal which looked like stainless steel: starting with gray, adding 10 pt noise, then doing a large motion blur, then unsharpen mask. By doing this several times, on different layers and then mess with the layer settings it will look like some kind of metal. Maybe. Other blotches and scratches you can do by hand. When you do them in an alpha channel and use that in render/lighting effects (if that is the english name of the filter) it will work out very well. Maybe. The result could be used as a texture in the 3D program of your choice. Or do the perspective transformation in PS. Ofcourse you can also use pictures of metal. Or trains.