The Steel Parts
Create a new layer and set it to Linear Burn. Here you will paint the base for the steel parts. The gold is painted on a different layer, also set to Linear Burn.
For the metallic shine, I use another custom brush, shown in the picture above. It's super good, because it shifts in transparency, perfect for this kind of stuff.
I brush in some light stripes of shine, and set the layer to Glow Dodge, then blend the edge with the Wet Bleed Blender. If I think it needs a little more, I go in and add it, it's really that simple.