Rusty Tutorial

1) make general selection
2) de-select non metalic items
3) create new layer
4) render plasma fog in selected area
5) use select by color to select random colored areas
6) remove selected areas
7) de-saturate plasma fog
8) use displace map on plasma layer
9) clean up plasma over areas that shouldn't have rst
10) perform bump map on plasma layer with plasma layer as the map (invert bump map will make it look more realistic, i checked tile by mistake)
11) set layer mode to multiply
12) duplicate layer
13) set duplicate layer mode to overlay (missed step in building this tutorial)

note, this is a generic tutorial, it can be applied to other similar effects such as mold, wood rot, scarring etc...