Enamel dull coat spraying from an airbrush

What kind,Modelmaster also,or Testers? Or something else ?

Linkhead, if you're talking about Testors' Dullcote (like this: Square bottle), then it does need to be thinned. That's the only stuff I've ever used. I thin mine 1:1 with lacquer thinner and it sprays dead flat. 

-O

I also use Testor's Dullcote thinned about 1:1 with the recommended 1159 Testor's laquer thinner. Lately I have not been able to find the 1159 thinner for sale. Does anyone know if you can use Model Master No. 2018 laquer thinner with Dullcote? I think they are both made by Rustoleum these days, but don't know if they are actually the same thinner. 

Testors has made both enamel and lacquer clear coats.  It is important to be able to decide for sure what you have.  Can you tell by smell?  The lacquer version has a quite pungent smell- the enamel version smells pretty much the same as the enamel paints.