It looking good Lis, @lis. I like exhaust stains so I would say your only add to the looks. Radials are dirty engines leaking oil and the lead and soot in the exhaust will stain the surfaces even on a well maintained plain. From the looks of wartime pictures it looks like the Pratt & Whitney Twin Wasp engines were a little cleaner that the Wright Cyclones on the B-17. The wing staining on the B-24 never seems as dramatic as the B-17. The images of dirty B-17's are easy as you can see below. The picture from Detail in Scale on the B-24 was the best and dirtiest B-24 I could find.
4 attached images. Click to enlarge.