Finished background

Finally, it's done. Well, this part anyway. The background is painted it. I decided to just suggest the details on top of the vector elements. They were painted on three layers: one in Multiply mode for dark areas and shadows, one in Color Dodge mode for specular highlights, then another one in Multiply for overall shadows. These layers were then taken down in opacity until I got the look I wanted. Because I wasn't putting in as much fine detail as in the foreground element, I wanted to knock it back a bit so it wouldn't compete. I also wanted to make it stylistically a bit different so that the pipe and threader would stand out. If you think the right side looks a bit empty, you are right. Another element needs to go there, which is another reason to keep it simple.