As you can see, it's all based on my favorite Enhance_AM combiner; AM_Dirt. My only problem with it is the way the blend percentages work. I had assumed that if you wanted an equal distribution of six evenly distributed grey levels (0, 51, 102, 153, 204, 255) then the blends should be five bands with the same limiting percentages. A look at a histogram of the resulting material showed this wasn't true. So after some trial and error I found the blends shown that produced a reasonably even histogram and I use these consistently with colour choices that are subtly different so they combine well to my eye. Using AM_dirt, I make the basic surfaces, steel and dirt (light and dark). CellTurb is used to combine a layer of light dirt with distinct dark spots scattered throughout. I also let the dark dirt slightly bleed through the light for even more variation. AM_Bozo is the combiner used to make regions of steel and dirt. The key variable here is the Gain of 99.99% which makes the dirt have hard edges. The AM_Dirt Bump variable is cranked up for both dirts but set to 0 for steel as I preferred the look of a separate texture material not based on colour variation.
Both the steel and dirt texture are variations of the Dented plugin from Alibi with the steel texture being small craters and the dirt texture being larger bumps. These are combined using AM_Bozo again with its variables the same as the colours so they match regions.