Here are some pics of F6F's that show some gun powder residue on the top, bottom, and leading edge of the wings.
Pic 1 is from Osprey's "Hellcat Aces of World War 2." Not the best scan, but you can kind of make out the residue. It is circled in green.
Pic 2 is from the title page of the same book. Again, residue from the muzzles is circled in green. If you look close, you can see it above the muzzles. The red circle surrounds where the ejection chutes are.
Pic 3 is from MetroBooks' "Fly Navy." The F6F is on fire, but you can see the muzzle residue on the top of the wing again. This is an F6F-5 with the glossy sea blue finish.
I think a lot of the confusion is that many pictures that you see are of planes before they head out on missions. Prior to going on a sortie, many planes were washed (at least according to several stories from aircrew talking about waxing the wings and such). Makes it tough to see something that is very faint to begin with (unlike the exhaust stains). Is it overdone? That depends on the plane and time frame. Some planes are immaculate. McCampbell's Minsi III is a great example of this. Others are anything but pristine. I have a picture of Jimmy Flatley's F6F on the Yorktown that is absolutely filthy.
I think it ultimately depends on persoanl taste. I think a little gun residue and some exhaust stains are what make the planes look like they are actually used, and not som restoration or museum piece.
Anyways, just thouht I would share some stuff I have found.