My favorite weathering application is spraying semigloss black then dry brushing silver paint on the entire piece. It really looks like a used metal piece.
View attachment 1493098But if your really want a gun metal look, spray the piece with FLAT black paint then rub graphite powder on. That is my go to for gun metal
View attachment 1493095So for the parts that are already flat black, just rub the graphite on that?