Some new official info from ubi forums

[David603]

Originally Posted by Feathered_IV

To my eye the above example of weathering looks quite poor. The random splotching with a photoshop brush and the childlike exhaust streak are at odds with the jet black/neon white national markings. Certainly not on a par with the best 3rd party skinners of Il-2.

Here is the other picture Luthier put up to show the contrast between full weathering and no weathering. Comparing the exhaust stain to RL pics of heavily weathered Bf109s, the color is right but its too well defined and in RL the staining follows the top of the wing instead of the side of the fuselage.

Don't forget the weathering is a slider though, not an on/off switch, so the weathering doesn't have to be so pronounced.