Marty Sanford developed a method for creating realistic exhaust staining on the F6F Hellcat. This is his writeup
I have been working lately on how best to replicate exhaust staining on the current model on the bench, the Eduard 1/48 Grumman F6F-3 Hellcat.
Due to the location of the exhaust stacks on the F6F and the high content of lead in American 100 Octane aviation gasoline during the Second World War, exhaust stains were very prominent on all aircraft using US Avgas. This was particularly noticeable on Hellcats in service.
Early F6F’s were painted in the three-tone scheme of Dark Sea Blue, Intermediate Blue and White.  This paint scheme was notorious for fading, being particularly unstable in the tropical Pacific Theater. Wartime period photographs of the Hellcat in service bear this out. Photo’s from most angles readily depict the exhaust stain running along the fuselage and over the wing area.  For both these reason, the Navy and Marine Corps later switched to an overall Glossy Sea Blue paint scheme for combat aircraft, which would remain as the standard well into the start of the jet age, or about the mid 1950’s.
A big thanks is due to David Hansen whose insights and correspondence into the pattern and color of F6F exhaust stains helped tremendously with this project.  It should be noted that a lot of this information should be equally applicable to other Pacific Theater Naval aircraft, such as Corsairs and Dauntlesses and Avengers (oh, my).
It took me about an hour on Monday night to airbrush the stain effect shown in these photographs.  I used Tamiya acrylics: XF-20 “Medium Grey”, XF-64 “Red Brown”, XF-69 “NATO Black” and XF-1 “Flat Black” all exceedingly thinned with isopropyl alcohol.  This has to be a very gradual process to look right.  The aircraft did not look like this after being fired up once, so think of this as layering in color, to replicate the effect of heat and gases as they discolored the aircraft. The paint “magic” has to happen in the same way. Only without the same fumes, heat, etc…
To quote David’s notes on this topic:
“1) long, very light gray stain first. Stain is attached to the fuselage just aft of the stacks and goes over the wing, where airflow is laminar and then trails away about maybe 2/3 of the way back from the leading edge, as the airflow changes from laminar to turbulent.
2) Brown exhaust stain: extends back to maybe the wing front spar; don’t know if this is partially burnt oil or some other kind of lead residue.
Very Dark Gray: begins immediately coming out of the exhaust jets and extends back a little ways. I presume this is all carbon.”
I placed a piece of Tamiya yellow tape on the cowling, flush with the rear vertical line of the cowl opening, adjacent to the exhaust stubs.  That would keep the exhaust spray from going forward onto the cowl (which won’t happen so long as the airplane continues to thrust forward, not backward, theoretically speaking). It also gave me a handy place to start my spray test the narrowness of my spray pattern. Cool, huh?You’re welcome.
I think of the exhaust pattern as if it were in a flame shape. You know what shape a flame looks like, don’t ya? Like a tear drop. Think of this as an elongated flame, in almost concentric colors, drifting aft from the stacks in a horizontal direction. The light gray, XF-20, mixed with a little white to lighten it, is sprayed first. Build this in layers, and not always the same area on every pass. Photos of Hellcats have this ending aft of the wing, right to about where the diagonal stripe is painted that shows the way to the hand and foot holds are located for boarding the aircraft. This should also extend from the fuselage sides onto the wing walk on the horizontal surfaces on top of the wing stub, inboard of the wing fold line. I could easily have dirtied up this model and it still would have been accurate.  What I am trying to achieve here if the “impression” of the exhaust fouling
without having it detract from the model otherwise. I hope that makes sense; it does to me in my bubble world.
What you want here is filth; but highly thinned filth. 15% paint to 85% thinner, maybe. Don’t allow any of these thinned colors to pool on the surface of the model – if you do, they may dissolve the previous layer. Remember, we are spraying acrylics here people. Dial back on you paint / air mixture, make short squirts, and always keep your airbrush moving.
The brown stain Mr. Hansen spoke of above is next. I used a murky mix of Tamiya XF-64 Red Brown. Contaminate this with black or grey or both, as I did, so long as it remains sort of predominantly a lovely brown yuck color. Use the “flame pattern” concept, and try to keep your passes inside the grey you had sprayed previously, extending no further aft than the main wing spar, just like David pointed out above. I found that if I made a mistake and “colored outside of the lines” on a pass, I could go back over this again with the grey; Don’t worry, the earth will still rotate the same direction on it’s axis just as before.
The same process is repeated, but with a yet smaller pattern for NATO Black. This is best described as an “off black”, or Scale Black for those who remember Pactra. Thin the paint as described above, use short squirts, keep the ‘brush moving and try to stay inside your small target area. When done with the NATO Black, mix in a little XF-1 pure Flat Black into your airbrush paint cup of filth, and turn the direction of spray around and paint the exhaust stubs, and trail the paint aft just enough to hit the center of your Carbon soaked burned fuel area. You still have that piece of yellow Tamiya tape protecting the cowl forward of the exhaust line, right? 100 Octane. You can almost smell the burn, can’t you?
You might have to touch this up – I did a few times. I also attempted this staining right over the top of my existing paint work, which is a little daunting. Just go slow and it ought to go OK. I do recommend you practice this on an old kit first. I have an Otaki Focke Wulf on my airbrush stand that I regularly abuse with this kind of trial & error work. Trust me: trial and error can provide an enormous teaching opportunity. You can ask me how I know this over a beer sometime.
So again, I hope that made sense. Remember, sometimes less is more.