Depends what kind of effect you're after.
If you're looking for a worn or used look, I'd recommend a matte clear coat. Testors Dull coat is probably my "old reliable" for a flat finish. It comes in a small rattle can, and it's lacquer-based, so it's pretty tough. Be sure to use a few light coats instead of one heavy coat. I had an issue years ago when using it on a model where I sprayed it on too heavy and it started eating my base coat! :facepalm
Another flat coat I really like is Polly Scale clear flat. It comes in small bottles and is applied by airbrush. It can result in a more 'faded' look to your colors.
Just a couple of ideas for you.