Chalk On A Hot Tin Roof

Today I reached a milestone on the produce warehouse - the roof is DONE, 100% finished.  In my previous post, I showed you how I made the corrugated tin panels which make up the roof surface.  Bright, shiny, aluminum foil pieces.  Pretty, but not exactly what you want for a realistic looking roof.  So, over the last couple of days, I've made it dirty, very dirty.

I started by drybrushing some streaks of Rust over the entire roof, followed by a dusting of a rust-colored chalk.  This photo shows one half dusted with the chalk, the other only has the drybrushing finished:

To me, this looks a little too red, so I darkened it considerably by adding a layer of brown and a layer of black chalk dust:

Much better!  A little more work, and the entire roof is done: