Here is a simple way to make some rust.
I used it on some Nurgle weapons, but it can be used for anything you want rusty.
Mix up a good orange and brown color and mix in a small amount of baking soda until it gets clumpy and paste like, then just dab onto the surface.
Very simple and adds color and texture.
After John told me about this method, I used it on a base for some texture, it works excellent to create little mud clumps. Good stuff!