To rust it up, I undercoated white, then base coated it in medium grey.
I then got some brown wash (agrax earthshade or devlan mud) and added some bright orange paint to it, to make a muddy orangish brown wash. I then washed the whole figure.
I then drybrushed with a large flat brush, with the bright orange, over the whole model. I then drybrushed again just on the raised surfaces.
I then put the black wash (nuln oil) over it, to bring out all the detail.
I finished it off with two more drybrushes of bright orange, on the raised surfaces.
(I forgot to shine an extra light on the figures when taking photos, so sorry if they're a bit dark)