I build in 1/700 and, quite honestly, use the RustAll system to rust the buildings and roofs in the dioramas, not the ships, because it's too hard to control the results in the small areas that I need it for in the ships. Even a total rust bucket isn't completely all-over rust. I will use either paints - most often judiciously dry brushed - or pastels, as wing_nut suggests. Depending on the scale, I think either would work well.
Here is an example of drybrushing, using Testors rust (1185), flat brown (1166) and orange (1127) and Model Master Rust (1785). Of those, the flat brown looked the most like rust when drybrushed:
Here is one that was given a few rust streaks with pastel chalks, rubbed onto a piece of paper and then transferred to a paintbrush and streaked down the sides of the hull where you wanted the rust to appear. The effect is a little subtler and in my opinion better if you just want a little rust. I use oil-free pastels in shades of brown, gray, rust and black:
Here is the last project, the only one I used the Rust All system on, the warehouse roofs. The ship isn't very rusty because it's brand new:
