You can use weathering powders in many ways. There are also alternatives to regular powders; Weathering pencils and now liquid pigments. These two can be mixed with water for less dry looking effects
I'd use a few different pigments, not just grey but also two more muted colours, say buff and starship sludge. The trick IME is the fixing part, without pigment fixers the end result will diminish over time, but using pigment fixers also changes the effect of the pigments. Heard it's maybe better to apply pigment fixers to the surface first, and only then apply the pigments, as doing this the other way around is much more chaotic. White spirits can also be used with powders to obtain various fx
you can just apply pigments with a brush directly in situ or use some sort of sprinkling method, like flicking a brush with pigment powder on it. Be sure not to apply too much pigments, better to work in a few smaller sprints than doing it all in one go
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