Guys, with 27 years working on the real thing, let me elucidate for you (ooer!)
If a military aircraft doesn't leak, it's empty, and they're never empty! We had brand new Tornado aircraft flown out to Germany in the eighties, they'd only had test flights, something like five flying hours on them, & they were dirty! And paint was flaking off. Okay, they weren't very dirty, but enough to show, especialy on the tail, from the reverse thrust exhaust. In WW2, a brand new aircraft to the squadron would have been flown from the factory, to an M.U. for radio fit/mods etc, test flown, then a delivery flight. So maybe a bit of exhaust staining would be okay. helicopters can get particularly dirty, especially operating out in the field.
You don't need an airbrush, I used one years ago, didn't get on with it, & have been brushpainting for years, just thin the paint before applying, the secret is, you need it thick enough to hide the colour of the primer, but not the detail. If you put on too much, Micromesh it, this tin's & will make it smoother. For
weathering, see the techniques section for hints on drybrushing, it's easy, it's also easy to overdo it, practice on an old one first, look at photo's of the real thing, see where they leak/stain etc.
It's a funny thing, but a factory fresh model looks like a factory fresh model, but well done dirt can make it look a million dollars!