I have one question for those who don't weather their model a/c.
--What colour do you paint your exhaust stacks?--
I have worked around real a/c enough to know that no matter how much we tried to keep her pristine, she did get weathered (oil, exhaust stains, paint chips, tyres turning grey, etc).
A museum a/c is a different story. I prefer to model reality, and believe me, a/c in a combat area don't get the TLC that a warbird does today.
On the other hand, I have seen some very over weathered model a/c too. One chap did a Spitfire up and weathered it much like an armoured vehicle. Be careful to know when to stop. I suggest refering to photos or even to an actual warbird at an airshow. Just ask the ground crew about the oil and exhaust stains, they'll be glad to tell you all about it.
Cheers