There is some question in my mind about these heavily weathered A6M's ... showing substantial amounts of aluminum skin beneath the green paint. For example, Tom Tullis' profile artwork in the Osprey Aircraft of the Aces 22 shows an Model 22 A6M3 of the 201st AG (Kokutai) with extensive aluminum exposed beneath green paint, whereas a photo of another A6M3 of the 251st AG showing heavy "weathering" explains that these planes were delivered to the theater in the factory grey-beige paint, where "hastily applied" green paint was used to camouflage the aircraft. Therefore, the exposed areas should be a "caramel" color per the latest research into the A6M: [link=http://modelingmadness.com/earlya6mcolors.htm]Early A6M Colors[/link]
I have seen no photos of an operational A6M5 (the CMP variant) with this same degree of "weathering" as found in photos of the A6M3's in New Guinea despite some fanciful profile artwork to the contrary. My best guess would be to follow normal weathering conventions regarding paint fading and chipping.
PJ