As a long-time modeller I agree with Leadmen. If you need to decal use a gloss, at most "egg shell" finish, apply the decal and recoat with flat. Applying a water-slide decal over a matte (flat) finish will generall trap air, leading to "silvering", caused by tiny air bubbles caught in the finish.
I generally use Testors' DullCote, most often out of a spray can. It's available in most places, it generaly works well, and usually doesn't attack fully cured paint. Use a little at a time, too much creates a milky final finish.
Someday when I get around to shooting pictures, I'll show some of my Airfix Multipose figures done as an MP and a medic. Both have their helmet markings done as described above.