I don't feel you need a layered shader to do this, you could do it with a simple blinn with the correct maps. As for using procedurals: they wont offer you enough local control over what you need to do. I would use painted spec, bump and colour maps. Get all the rusty info painted into a nice large colour map, using real stock photos of rust http://www,image-after.com
(obviously!) then knock your spec right down and your reflection right down. Add your dirt maps for those two parts and start to add them back in (remember that metal is all about the reflections so get your environment all set up and looking nice, use a chrome ball shader if you need to see how it's looking). Add your spec back in slowly looking to keep it to a minimum, do the same with reflection. You can add your bump or displacement after that .
It's all about the subtleness in the texture so add back in in small amounts
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