I don't like the way you do the metal. It doesn't quite seem.... "natural" -- and mind you I find it hard to put in words why, and exactly what a solution might be. Maybe it's just that the amount and the pattern don't seem right. Especially the part where a line of speckled chipping will just follow a panel line for no reason (see the gun bays, see aileron/flap lines).
In regards to the details: See here for a little debate about some of the minor details, including the serial number:
http://www.ww2aircraft.net/forum/aircraft-picture-requests/tex-hills-p-51b-mustang-4403.htmlThere are differing thoughts on some things like if the eyes are red or black. Looking at the photos I think they are black, and the supposed different color is really just where that next panel makes a dog-leg kink and reflects light differently. However, don't forget the stripes on the tail! Most sources seem to show a variation of them:
http://media.photobucket.com/image/%2522tex%20hill%2522%20%2522p-51%2522/280163/P-51B.jpgIn regards to the amount of chipping: There doesn't seem to be nearly as much paint loss. Just around some of the key engine panels. Right around the canopy where folks lean against the frame, maybe as they kick and scrambled to get the cowling off that's how the forward section lost that much paint on the port side. The rest of the tail wouldn't just flake off unless people were kicking and stomping on it, and it's not a walkable area, and it's not an area that would receive wear and tear. Not like that.
See here (HUGE photo! You've been warned!)
http://www.nostalgicaviation.com/Hill%20Tex%20on%20P-51%20wing%20root%20climbing%20in%20cockpit689.jpgAnd a different photo here:
http://media.photobucket.com/image/%2522tex%20hill%2522%20%2522p-51b%2522/LWMTC1/Hill002a.jpgI also think that the leading edges are not paint chipping and metal showing through, but rather yellow or white stripes wearing off and green showing through. This is the exact spot those stripes were located. They would be field applied and more susceptible to wear than factory applied finishes. You don't see much actual paint chipping outside of the panels immediately by the wing root and the removable engine compartments. There is no chipping on the underside greys of most US planes. Maybe it was the quality of the grey paint or the fact that it's upside down so nobody is stepping on it all day, nobody is dropping panels on it as they remove them, etc.
I'm going to be critical of your underside dirt, but please forgive me it's constructive criticism...
The underside looks like it took a belly flop into a mud pool. In fact I've seen photos of P-51s that bellied in and they don't look that muddy! It doesn't look right. How it would look right: If the splashes were specific to the wheel areas, and the "throw off" zones of spinning tires. It is also just a blurry shade of brown, which is too vague IMO. You would get a better weathering effect if it was more specific weathering. Instead of a blanket area, smaller areas of specific weathering layered on each other. I.e. oil around key points you've seen oil leaking from on the real deals, dirt throw-up from the tires, under the tail as well ,but not the doors, they would be open and the insides would get muddy! (this might be interesting but I've never seen a plane where the mud was "masked" slightly where the doors canted outwards). Oil or chipping around the bomb racks which might see more banging while being serviced. Things like the making the screw-down bolts of the removable engine panels stick out on the underside (give them a little "pop" against the grey belly color). Oil dripping (or even just semi-translucent dark liquid) out of the radiator duct exhaust panel lines.
That's just a laundry list of suggestions to improve it, off the top of my head. I think you can't go wrong with actual real-war examples. However, one thing you need to change is the materials.txt file. It's somewhat shiny, and not befitting a more worn aircraft (which ought to be more dull?). See if you can pull the materials.txt from the G4M1 Betty or B-25H Mitchell default. That might be a good fit.
