Thanks; I've gotten a few inquiries as to the painting process. I'll paste a response I made here:
The gun was first primed and them spray painted with several coats of different colors. I used duplicolor automotive paints. First with silver, and then black (with some hints of metallic in it), and an off white color. Every coat was completely dry before I sprayed over it. I did this because i would sand and chip into the surface to reveal layers beneath. The gun was not disassembled (except for the part underneath the front as it will be modded), just masked.
One way is to load a brush with some water, with some paint and maybe some matte medium, and then cover some of the surface with it. Then you can take a paper towel and sort of "dab" the surface while it's still wet; this can create an illusion of grime that has settled over time. Some medium to light grit sandpaper over this can help to diversify the surface more.
What I did on this gun was work in layers. That's usually how I like to do it, as you can sand through the layers of paint to reveal the layers underneath. So if you see some of the blackish marks on the white parts of the gun, that's just black paint underneath the white that was sanded or chipped away. This can add a lot of subtle depth. The brownish grime you see is simply acrylic paint mixed with matte medium and water, that was brushed on, and then when it's starting to dry and still flexible, you can actually use a tool, sandpaper or even your fingernail to kind of scrape some of it off. This can also create a really convincing effect. I then took an exacto and lightly dragged it over the paint in some spots, and that can also help to reveal the layers underneath. If you have a razor blade, or one of those box cutters, you can snap off a piece of that and use it to carefully and lightly scrape away some of the surface, as the blade can sit flat and shave away layers that way. But be careful as to not scrape down to the raw material (in this case cheap plastic), unless that's what you want.
It may take some trial and error to get it right, but the basic thing is to think about it in layers. If you think about it, real natural weathering is basically that! Just years of layers of grime and damage to a surface. This is just an imitation of that.