Some tips:
Create your Brushed Metal Layer, add a Sepia Tone to it, then create a New Layer, add some brush work to it with something like "Scratch and Dirt" brushes found in the Resources Section of BioRUST. This'll give that Brushed Metal a "beaten and used" look.
Each scratch or dirt brush work should have it's own layer, that way you can adjust individual settings per layer such as opacity and the like.
Also, take a close look at one of my wallpapers, see the resemblance to rusted or beaten up iron/metal?
http://forums.biorust.com/photopost/...0&ppuser=28595
This was done by applying a chrome/metal Layer Style to a brush I made.
1) Lay down your brush and add your Layer Style
2) Duplicate that brush
3) Simply erase parts of the Layer Style using one of "Spatter" brushes in the DEFAULT brush set.
Erasing different sections of Layer Styles is a very handy technique - something I've found on my own.
If that doesn't do you some justice you can always Google a tutorial on more methods.
Hope this helps, enjoy!