I'll catch lots of flak for this, but I do like to wash over matte finishes. The techniques I use are mainly for armor but will work on aircraft (I used to build those too before I realized my armor models always turned out better). You don't want to be doing this on the exterior of your airplane but it'll look good in the cockpit, weapons bays, landing gear, etc.
1. Paint your surface whatever color you want it. Tamiya acrylics usually dry very matte and almost chalky when brushed on.
2. wait a day or two for the paint to dry
3. get some artists acrylics (these are not the same as tamiya paint) they come in small tubes or plastic squeeze bottles, my favorites are Folk art and Delta Ceramcoat, get yourself some black, a medium grey, and any variety of rusty browns (burnt sienna, burnt umber).
4. With the acrylics diluted in water and applied over a matte finish, you'll be able to do an initial sloppy wash with your darkest color, try to push the watered-down paint into corners and around raised details, as it begins to dry, you'll be able to controll it a bit better and use it almost like a gauche to add a tint, or clear film of paint over the original surface, this will make the cockpit seem more "used"
5. when this is dry you could add a second, more controlled wash to bring out your details, the colors will blend a bit once you rewet the old wash, but thats ok.
6. if a droplet of paint./water dries in the middle of a flat surface, it'll leave a stain, you can rub these out with a damp Q-tip.
I know it sounds a bit iffy when explained w/o photos but try it out, you said you build cars, try it on the seats or floor carpeting.
For the main surface of the plane you wont want to use this technique, for that you should go ahead and apply a clear coat first. Use a wax floor polish, like Future (or whatever they call it where you live). then do your wash, and finally apply a satin finish coat. Aircraft aren't really matte, even a zero out at sea, wouldn't have a compleatly flat finish. Also be aware that there are some folks out there who won't do a wasgh on aircraft, because they think it over-accentuates the panel lines and since they're already out of scale.....yadda yadda, its a question of artistic vs. realistic. Matter of taste really.
Hope I wasn't too confusing,
Ralph