Just an artistic suggestion, so take it for what its worth.
First, the bottom looks a little bit too "blueprinted". The difference in color shades is too dramatic. But maybe that is just my taste.
Second, I would think that weathering would be along or within individual panels, rather than across "seams". So, for example, a patch would stay within a panel. If damage was so severe as to cross a seam, then I would think that the whole piece of the damaged panels would be slightly different shade from the rest of the bottom of the plane - in short, the different color would be the whole square of the panel; or, alternately, just a small dab of paint over a bullet hole.
Third, (and my biggest concern) it always seemed to me that airplanes launching from grass strips would bear a lot of weathering along the bottom of the landing gear doors on the gear struts, particularly along the lowest part of the door, since this is where dirt and scrub brush would be most likely to rub against the airplane. Yet, there is only a small amount of wear on the gear doors of your 109, with most of the wear on the centerline of the fuselage.
Just my opinions. Not that I'm an expert or anything.
