I use Testor's Dullcote for a really flat finish but lately have found that Vallejo's Polyurethane Matt Varnish # 26.651 works just as well and is compatible with decals and lacquer, enamel, and acrylic base coats. It's a great product and thinned with their # 71.161 Airbrush Thinner sprays beautifully at about 20 psi on the Airbrush.
Cheers,
Lance