Do you mean the lacquer Dullcoat or the Acryl Dullcoat. I use MM Lacquer thinner for thinning and cleaning when using the lacquer Dullcoat. If you're using Acryl Dullcoat, you really don't need to thin it; it's airbrush ready right out of the bottle. If you do want to thin it, I recommend thinning with MM Acryl Thinner and cleaning with MM Dried Acryl Cleaner.
Jesse