| As Nigel says rust is very much dependant upon a lot of things such as the model you are making, scale etc..
I have always been a great fan of dry brushing but I have also used airbrush and pastels for creating the effect.
I have seen one or two models with real rust used for weathering and I have to say I don’t think it works very well. The main difference being that rust is actually multicoloured when you study it so applying a powder of real rust does not tend to show this. I have studied a lot of rusted surfaces over the years and have actually taken pictures of various rusted items on ships, much to other crewmembers amusement, and I am convinced that using two or three colours is a more effective way of achieving a realistic effect.
As with so many things in modeling try experiments with various finishes and see which you prefer. I have attached a picture of a destroyer in 1/96th scale which was a mixture of three different Tamiya colours , purple, brown and bright orange which although I admit looks too much for this model, I will be using the same technique for my steam coaster model in 1/35th scale. | 
Linear Mode
