A lot of folks are getting into weathering powders for stuff like that now. Forge world just released a set of powders. Basically it's just the pigment from paint and you can brush it onto your model and you get smooth transitions like that, and you can use rubbing alcohol to get it to stick or flow into cracks for dirtier looks. Other than that though, what you could do if you wanted something real rusty is paint it dark brown and then drybrush oranges onto it, followed by boltgun metal for some parts to show where the metal isn't rusted yet, or where the rust was scratched off and you can see the bare metal
the best weathering tutorial I ever saw is here:
http://ultrawerke.blogspot.com/2007/03/painting-and-weathering-tutorial-part-i.html
about halfway down on page 2 he starts doing rust on the tank so that might give ya some ideas.