Hey frank-
What is the overall effect you are looking to achieve? There are a variety of techniques that you can try... it all just depends on the look you're going for.
If for example, you want to have a faded decal look and you're working in a small scale like 1/72 (my personal fave) then you may want to make a thin wash of Arctic white, or antique white to cover the decal... just make sure it's not to opaque. If you're working in a larger scale of 1/48 or above, using a stensil to paint in your decals becomes more realistic.
Of if you're just asking how to apply a dull flat finish, there are plenty of spay on materials that give a nice "dry" look. I actually like using Citidel's dull clear coat on my aircraft. (See pick below of my Hellcat Mk. I... it's a 1/72 Italeri kit)
Gear's suggestion to check out FineScale's web sight is a great place to start. They have a BBS like this full of guys who are quite skilled and eager to share their knowledge.
I also like
http://www.armorama.com they have a good Air-craft section that I'd suggest you check out...
Best of luck!
-scott
