Hi, here's a relatively simple method to paint rusty metal. I learned that there are two important elements: the rust must not appear as a uniform stain, and it must appear to have a nasty, gritty texture. Here's how I did it:
I undercoated the area with black, and applied several layers of watered down Leadbelcher (Citadel) to produce a nice, even layer of metallic paint.
Here comes the rust:
1: First the metal was given a wash of Agrax Earthshade (Citadel)
2: Spots and stripes were made with several layers of Carroburg Crimson (Citadel)
3: A couple of layers of Bloodletter (Citadel) were added, slightly bigger than the carroburg crimson spots, so that there is a gradual transition from bare metal to the reddish area.
4: I then used a thin, old brush to apply tiny, crusty spots of Rust Effect (Modelmates) in the middle of the stains on the flat surfaces (water would pool there, severely corroding the metal). I don't smear it on, instead I jab the thin brush at the area to create a random pattern. Once it's dry I add some more so that the rusty crust will be nice and thick, a little craggy landscape of corrosion.
5: Finally I drybrushed the crust of the Rust Effect spots with Ryza Rust (Citadel), and added streaks of watered down ryza rust to the middle of the vertical rust stripes.
The trick is to remember that rust will form where water flows and pools, especially around rivets.
Here's the result: