Weathering is easier than it looks first prime your model in a dark brown then if you have an airbrush you can go back and hit areas with a lighter brown (optional) I just like multi color rust

here comes the trick HAIRSPRAY it and just like paint 2-3 real light coats will work best try not to pool it... let completely dry, now paint as normal. after it dries use a large wet brush to hit areas you want to weather then scrape where you want the damage. I use a broken pair of tweezers (one half shoved into a
bic pen body) then the final step is to highlight (well err opposite of highlight) use a black pastel powder and hit the cracks corners and around nuts bolts hinges or anywhere that may need a shadow. you can make this cheaply using a black pastel chalk (non oil) and some sand paper..... (have a dedicated brush for this)