I find that for this sort of work, Tamiya Smoke works well. You need to dilute it with Tamiya acrylic thinner, 3 parts thinner to one part paint. I also add a few drops of acrylic flow enhancer.
I then apply the diluted paint with a flat brush. Not too much at one go, just let it flow into the crevices and recessed detail. Let it dry for an hour or so, and repeat until you have achieved a sufficeint degree f shadow. You can see the effect on the main gear legs of this Airfix 1/72 TSR.2:
Tamiya Smoke is glossy, so you mihgt want to tone the effect down with a light overspray of matt varnish before installing any weapons.
Alternatively, a dilute wash of burned umber watercolour, with a drop or two of liquid soap added, works well, but is a bit more fragile.
Cheers,
Chris.
Cute and cuddly, boys, cute and cuddly!