Metal: now the metal is completely different thing. It is very shiny. The shadow - reflection value must be very high: that meant no matter what the color of the metal is you will have to use white (top reflecting point) and black color (inner shadow). The metal surface is often very smooth - that we will do the rendering with a mere soft edged brush (maybe with noise turned on).
Going darker and darker:
Scratch the surface using basic 1 px brush.
Drop some light reflections using ROUGH brush. The metal has very strong counter light coming from the edges.
Gesso: now gesso has no wide shadow - highlight value. That means we will be using white (primary) and light
Cast iron: now the cast iron is black and has a very small reflecting area in this case. Moreover as this material has rough surface: that is why we have to use textured (dot texture brush) for the reflection.
Fill the selection with black color first.
And that's a wrap. Practice different materials. Because I thing the Texture photo usage is a little cheating. This is a good advantage for you to know how to render the materials using plain means.