Went a bit OTT with the filth. I’ll have to tone it down on the next one.
Lots of practice dry-brushing various combinations of paint and inks appropriately thinned and layered, and keep it subtle! It has to replicate the way it actually happens on the aircraft while in service; the plane starts off all clean and shiny, but flying causes oil, exhaust gasses, dirt and dust from the air itself to build up in very thin, faint layers, accumulating over many hours of combat flying, so instead of trying to dry-brush fairly dark paint in a thin layer, you have to gradually build up very thin layers of very thinned dark ink or paint, just kind of ‘sneaking up’ on it, gradually adding more layers where the staining is naturally going to be darker, inline with the stub exhausts, behind the gun ports and shell ejection ports, etc, and building up very thin layers of thinned silver along all the leading edges of flying surfaces, prop blades, etc.
Loading the brush, then wiping a fair bit off on scrap card, or paper* by brushing like you’ll be doing on the model, then brushing on the model!
It’s a time-consuming process, but it pays off if you don’t rush it; many hours modding and painting AD&D figures teaches you that, they’re a lot smaller than an average plastic kit!
*Old phone directories are very handy things to use for this, you can mix paint in small amounts on a page, wipe the brush to get the dry-brushing just right, then rip the page off and bin it after each session.
I’ve still got one of the really thick ones, with the heavy board covers, from when I used to paint AD&D figures and pin-stripe/sign-write friends cars and bike helmets, very handy thing to have. 😀