Thanks for your comments, guys!
I'm experimenting with a deeper shade to the aluminum, to allow lighter accents like scratches to stand out more. Generally I'm happy with the look, especially when I compare it to my earlier bare metal skins or AH's default bare metal P-38L skin.
Krusty - you saved an error from being passed on - the nose gear door was indeed not natural metal but left over from the earlier skin I made on the same template. Special thanks for your keen eye.
As far as the smudges-- if you are seeing giant black smudges, I've done something terribly wrong. But there are exhaust stains on natural metal, according to Guppy and this photo he provided a while ago - check out the port tail inner surface:

Pretty noticeable, aren't they? That said, I haven't looked closely at my intensity level for the smudging yet and am sure I have room to lighten it up some.
I am trying to get a dirty, streaky look based on some observations I made of unmaintained airfoils operating in alternately dusty/wet conditions. I think these streaks would develop on a painted or non-painted surface, but I can't say for sure. Just a theory and a look I thought I'd try for.
I may lighten the shade of the nose panel a bit, but I don't think its actually white as shown in one of the profiles - I think the artist was just using differing shades to show natural variation in metal texture- the other profile shows the shade as generally being the same as the rest of the a/c.
Thanks all for your comments and help. Still more than a page of changes to make to this one, and it looks like I've missed the skin pack release so no rush now.