I like the sound of dry brushing, I'll have to try it. Am I right thinking it's better just for making it look weathered and not specifically dusty?
I'd also recommend checking out Randy Jacob's 'Dirty Multicam' and 'Post-Apocalyptic' paint jobs for some techniques and inspiration. It's a YT channel btw. Looks absolutely awesome. Gonna try and make masks look weathered using a few ideas like the dry brushing etc
Depends on how you do it. You have to think about what it is you are trying to emulate whenever trying to achieve a faked look with paint. In this case, it is the notion that dust has settled in bits that are hard to get to, but has rubbed off areas where your uniform, hands, webbing etc have brushed against it.
Let's be honest, with something like an M16 Armalite type of weapon, you have to keep the real thing clean because it is manufactured to fine tolerances, unlike an AK, where everything is as slack as a bag of spanners, so dust won't easily jam it, whereas an Armalite will jam easily in those conditions if not kept clean, which is why the originals had a cleaning kit in the stock. But out in the plains of Afghan or the desert of Iraq, that's easier said than done, so you'd wipe the thing with an oiled rag and hope it picked up all the dust and kept it out of the mechanism. In that sense, you might simulate that look by, as others have suggested, painting sand colour on it and wiping it off so it only stays in the recesses, however, if you want to do something like that without resorting to slopping paint all over the thing and then wiping it off, then you could as I suggested, try painting it lightly sand coloured and then dry brushing the black gun metal colour on, which would only go in the easily accessible areas, so would simulate the sand and dust only being retained in crevices when the thing had been wiped with an oiled rag. Both techniques will do the job, but one is messier than the other, so it's up to you. Just try either technique on something else before attacking you pride and joy!.
There is another technique you could try too by the way, which is to let down a sand coloured acrylic paint so it is very watery, and then let that flood into crevices; when the acrylic dries, the pigment would only settle in recesses, much like dust would. Acrylic is much easier to sort out if you overdo it, since before it has dried, it can easily be washed off areas where you've gone over the top with a brush dipped in water or a wet tissue, but the acrylic when dry, will be waterproof. Acrylic dries much faster than enamel too, so it is quicker to get things done, and you can usually find a set of cheap acrylic paints in most supermarkets too, so it's easier to get hold of and less expensive.
Yet another technique is to use a clear flat varnish with some talcum powder in it, which will make the thing look a little lighter and dustier. Note that adding talc to paint has the effect of making paint more matt too, so it's kind of an added bonus.
