Hi everybody:
For doing exhaust/gun soot, I usually use ground pastels or pigments. And for the exhaust I usually use a mix of 2 parts black to 1 part brown, as to me the straight black is just to stark. Although on late war Luftwaffe planes I like a bit more black, and much heavier as they were using some pretty bad mixes of all kinds of things to get fuel to fly with, and they showed some tremendously heavy streaking. For anything with a very dark color (like U.S.N. dark blue) I sometimes like to put a very thin layer (almost can't see it) of white around the edge.
For the guns I usually go fairly light as most countries were using smokeless powder by WWII. But as can be seen in the photos above, there was still an accumulation after enough firing. And some pilots believed that shooting 500 rounds at an enemy was better than shooting only 50
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