It is looking promising FTJR. Here's a few suggestions I have you might want to consider:
The panel lines all look very uniform in opacity. Lines on things like the edges of the control surfaces, undercarriage, gun and engine access panels etc should be heavier than the other lines as they represent breaks in the skin rather than overlapped panels. Easiest way to do this is to create copies of your panel line layers, change the lines' colour temporarily and then erase the lines you don't want to be heavy. Ypu can then vary the opacity of your new layers to get them looking right.
The front of the elevators and ailerons could do with a thin shadow on their leading edges. Easiest way is to draw a one pixel wide vertical line, gaussian blur it, and then remove one half of it so you get a sharp dark line on one side that tapers away to nothing on for a few pixels on the other side. Now copy and paste copies of this line onto your control surface hinge lines, rotating them as necessary. The sharp part of the line is on the hinge line with the tapering part fading back onto the control surface itself.
WRT weathering I've found a couple of threads where I've discussed it before
here and
here.
I'd say don't be afraid to add more layers, my skins often have 20-30 layers of weathering effects. If you chuck everything on one or two layers you can't vary one effect against another later on.
Maybe add a second layer of oil streaks below the fuselage, but lots of thinner (one or to pixels wide) and slightly wavy ones, partly erased as they thin out towards the rear. Combined with the more general oil staining you've already done, these would look more realistic IMO.
On most planes the cartridge ejection holes usually have staining behind them as well as the gun barrels themselves, although I haven't looked at WW2 P-40 photos to check this.
