Almost everything you wanted to know about Dullcote:
Testors Dull cote comes in rattle can AND in a bottle for brushing or airbrushing. It has talcum powder in it to provide the dullness, this has to be stirred up before using. It can be thinned with anything you thin enamel paint with. I prefer the bottle (hehe) and like to thin it 50/50 with lacquer thinner and then airbrush it.
Come to think of it, you can really make your own with thinner and talcum powder, hmmm. You could vary the amount of "dullness" by increasing or decreasing the amount of powder. Hmmm. Not sure about this one though