Read more about How to paint a complex metal structure at CreativeBloq.com
Here you can see a concept I did few years ago that can be used to answer your question. Usually when I paint big structures I tend to use simple shapes that can be read very easily. I then start to render and add detail to make the shapes look more complex. I also add elements such as pipes, panels or any sort of prop to these structures, which makes the surface more visually interesting.