I like to use Testors dull coat. Well, it used to be called dull coat, but now it's
called "Lusterless (Flat)." Number 1960 in the spray cans.
It gives a dead flat finish. It is a lacquer, so if you spray any lacquer over enamel
or acrylic paints, give it a couple of lighter coats instead of a single heavy coat.
I've tried the Krylon matte spray also. But it still has a bit of a sheen to the finish.