If you pay close attention during the daylight to shiny objects or even semi glossy objects from metals plastics glass, etc you will notice that on the edge of the objects there this glare, shiny line which enhances the edges of the objects.
To do this you must crate white lines with a 3px brush right next to the black lines of the object we did at the very beginning. It is important these lines are visible and are right next to the dark lines. With a soft brush erase the edges of the lines giving them a gradient like effect like so and set blending mode of layers to overlay
You can see this again
on the cap also
This is what the white highlight lines should look like. Notice the gradient and notice they are right next to the edges of the dark lines
This is black drawing with white lines. White lines were done on the very end of the illustration
As you can see this is how the highlight lines look up close
Final. This took about 10 hours to do some over 300 layers. I could have done it faster but I was figuring things out and experimenting as a I was moving along. The gray background give the metallic illustration a distinct appearance which i really really like.