07-21-2011, 06:12 PM
To get that look You may try thin washes of grimy black applied with an air brush.
This set of train masters was weathered that way. Although not quite as severe as your "G" and in Steam power black not Brunswick green, That technique will give you a similar "gray haze" result with a few more coats of grimy black.
My other car is a locomotive, ARHS restoration crew