This shot demonstrates a couple of techniques I've used successfully to produce realistic rust. The first is the roof and power generator, which were treated with Instant Rust by Sophisticated Finishes. The second is Dr. Ben's Realistic Rust, which was used on the 50 gallon drums.
Instant Rust is a 2-part process. The first application is of a dark gray gooey "paint", that delivers real metal particles to the surface you intend to rust. It can be thinned but I found the effect was more pronounced when it was applied full strength. The second application is the "antiquing solution", which is a chemical that reacts with the metal particles to produce rust. Of course, it isn't as "instant" as you are led to believe. It does take several hours for the chemical reaction to start producing rust, and this reaction lasts several hours, or as long as it takes for the solution to dry.
Dr. Ben's Realistic Rust is probably the best of the 1-step products I've tried that produce a rusty effect. In the case of the drums, all I did was roll them around in the bottom of the jar (where the solids collect), and then pulled them out and let them dry. The solution must include some kind of binding agent, because it doesn't just flow off like so many of the other rust products. It also seems to be pretty durable - an alcohol/ink solution applied over it did no damage.