Weathering is new to me and it's a nice change every now and then, I would only do two or three models a year.
I don't use an air brush I like to use real soil, this does mean that you will have to be careful when handling the model as you will rub off the dirt.
I use various paints to get the desired effects.
I like to work from photos of the real machine so I can get the blemishes and marks where they are supposed to be.
I can cut bits off the model to give it even more life like look when heavy weathering.
I can do light weathering, not every machine has rust!
If you have an idea of how you would like something to look please supply a photo so we are on the same page.