It's not dirty enough! Seriously, the Skyraider is the one airplane where "too much is not enough." You want to airbrush really biiiiig streaks, then go back over them with thinned Tamiya smoke and let it "drip" down the side. The centerline tank should have so much oil on the front upper end that you'd be afraid to touch it. The R-3350 is one dirty engine. If you go over to Modeling Madness and look up my review of the Z-M kit here:
http://modelingmadness.com/review/viet/tc/tmca1.htm
You will see what I mean. This was done from photos of this airplane. You can go over to skyraider.org and see airplanes that make this look clean. I remember seeing them when I went aboard the Coral Sea one time back in 1964, and it was amazing how in the nice Clean Navy, those Skyraiders looked so godawful.
Like I said, an easy continuation of what you've done with this otherwise-excellent model. It's the one exception to "less is more" when doing weathering.