My methods used here are:
1. A cashier paper roll, which I sprayed the entire roll with Citadel black Chaos, my absolute best black spray primer on the market for modeling. I personally think so.
2. I brushed a mixture of light, dark rust, white, black, and grey pigments until I was satisfied with how it looked.
3. Then I took some odorless thinner and dabbed it on the entire roll covering every part of it. A tip is to not soak up the pigments too much. I went against this tip, only when I wanted a 'caked-up rusted' look, as in one of the photos. Let it dry.
4. After the thinner had almost dried up completely, I sprayed on a dull coat on the entire surface.
...and Voila!
