There are several methods of applying rust weathering to a model. The RustAll system works very well but is rather pricey (IMHO). Pastels will give you absolutely great representations of rusted areas, but the first time you dust the model after it's built, will be lost. And since a coat of clear painted over it will tone it down drasticly (would you believe disappear?), you have to learn how to "over weather" with the pastels to get the results you want. And lastely, there are just plain old paints labeled "rust". Again, there is a learning curve involved, as you are going to need to know how drybrush in order to get the subtle kind of rust patterns found on ships and boats.
Just remember that rust comes in many colors in nature - from almost orange in new rust, to a deep, almost chocolate brown in old rust. You can either mix your own or get bottles of rust paint from several different manufactures. I have bottles from Testors (the little square bottle), Testors MM, Humbrol and Testors MM Acrylics because each bottle has a different shade of paint to represent rust. Go out and look at a piece of equipment (car, truck etc) that has been rusting. You're going to find rust in several different colors in different places on it.
My suggestion would be to pick a system you want to try, play and practice with it on scrap or a "hulk" model until you are happy with the results and confident of using the technique and then apply it to your model.