Graham, do you have a soft brush that you use exclusively for buffing up the mr metal?
When I am representing metal components, such as Fokker fuselage frames, I drybrush with my buffing brush over the blue/grey paint, which does two things - on raised areas the metallic effect catches more, giving the impression of worn paint, and on the surface in general it gives a metallic tinge. As long as I keep on buffing up other mr metal components from time to time, the buffing brush remains charged with metallic pigments.
I just wonder if the same would work over a larger surface, such as you propose, to replicate the sibergrau effect?