Decalling 101, here goes!
This will not work without an airbrush, but:
When you've finished the paint scheme and fading/weathering (but NOT exhaust/gun/etc staining), varnish the aircraft using clear high-gloss acrylic varnish; Gunze diluted with a mixture of industrial alcohol and Tamiya thinners (NEVER water!) is best, as Tamiya tends to 'powder' if you're not careful. Leave the aircraft for a couple of days.
Apply the decals as usual, but if they are not Japanese made, setting solution is a useful thing to use, especially with recessed panel lines or raised detail. Don't use it with Jap made decals as you will not be able to slide them around. Aeromaster, etc, is great for this, and predictably responds really well, if you follow the instructions carefully.
When the decals are well day - another day! - run a brand-new razor blade very carefully and gently along the panel lines (if recessed) where the decals cross them. Using a fine paintbrush, run a drop of setting solution, whatever the origin of the decals, along the lines.
When it's dry again, rinse the setting solution off, and clean the model using a good brush and your airbrush as a blower to get the crap off.
Varnish it again using the same gloss varnish.
Now you can bring out the panel lining by running a drop of thinned down enamel paint into the panel lines; wipe the excess away with a bit of clean rag which you have moistened with a drop of white spirit.
Let it dry again, blow it clean again, and varnish overall with a decent enamel matt varnish, spraying very thinly to avoid overdoing it, which will cause whitening of the varnish.
After all that, you can now add ehxaust stains, fuel spillage, etc, bearing in mind that in real life they can dirty the insignia like anything else.
Hope this helps - 'spect you knew all that, but I'm sure somebody'll find it of interest! 8)