Think about it in layers. Your very bottom layer of your body will be metal. Likely dark, dirty brown rusted metal where exposed. So if I am starting with zero paint, I primer gray, then primer red in light coats. Sand between coat, make it tool terrible in spots. Grimy black, rust, dirt. just rough it all in. Very thin paint. In between coats, get a set of weathering chaulks. Then grind up some dark brown chaulk dust and dab a small, very fine horsehair brush into the chaulk and very lightly flick it at your model froom a couple feet away and let it fall naturally into crevices between paint coats. If you have an airbrush or two, very lightly set the dust in place with matte finish, thinned about 75% or so. Just dust it. Same as the chaulk - spray into the air from a couple feet away and let it settle on your model. You can also use hair spray.
There are so many ways to do it, but you will usually see the good ones are done in lots of stages, bringing the effect in slowly. Dab some paint here, springle some rust or grime dust here and there, etc. If you set the dust and the matte (or hairspray) lands on your model looking like dust as well, you are doing good. You don't want to see shiny matte or hairspray anywhere onless your creating buildup for chiseling a rust pocket into and you need an inside shadow around the edges.
So basically, rough it in, then start refining the details using lots of light layers. It is never the same for me. Every model I weathered in my past model railroading life was done with different techniques, simply because each model was different, sitting in a different environment on a diorama, and even creating these looks on supposedly different materials.
Research helps too!!!
For example:
Two ways to go...
1. Paint back to original, but completely flat paints. No rust. Then apply rust, building the textures layer by layer. Dust, streak, wash, rubs, more dust, etc. Each dusting, try to get a very dry coat of matte clear applied. You want to build up an armor shell over you textures. Then for your very last detail, Stand back with a bottle of hairspray and some loose, soft fluffy clay dust you found outside
2. Start from a white styrene or ABS body. To make it nearly an indestructible finish, see if you can dye the plastic dark brown, then a dip into black dye. That way if you scratch the finish, you're exposing yet another layer of rust. Same dusting, wahes, dusting etc as above.
Food for thought...