Here's my first attempt at rust you might want to try:

1. Liberally brush on Aqueous Hobby Color "Rust" acrylic paint or equivalent
2. Before the rust colored paint dries, sprinkle on some talcum powder, corn starch, baking powder or other similar powder
3. Dab on Tamiya XF-10 Flat Brown (covering some of the powder that hasn't taken on the color of the rust)
4. Wipe on some black to simulate soot
5. For added effects, Tamiya sells a "Weathering Master" kit that has some bright Orange rust you can apply for highlights

...hope that helps