Here's an overview of how to bake from High to Low in xnormal.
After you get all your bakes done, that is basically your "Base" You can hand paint on top of that in photoshop.
Or what I commonly do for more realistic stuff is use photosource (in this case, images of metal and leather) to overlay a basic pass, then paint over that until done.
So - Base Bakes - Photosource pass - Final painting pass
Hope that makes sense.