I have been using an airbrush to basecoat the tanks for my Imperial Fists. The foundation paint I'm using is Iyanden Darksun. After around 15-30 minutes of use, the flow of paint starts to lessen, and will eventually stop. To fix this, I have to run a paint thinner and water through the airbrush, before going back to my paint mixture.
I am adding enough water to the paint. As a test, I upped the ratio of water so much that the paint was running off test surfaces. The airbrush still ended up having trouble though. The helpful, knowledgeable, and friendly hobby guru at the hobby store suspects the foundation paint is the problem, as it likely isn't designed to flow through an airbrush. He is familiar with GW paints, but hasn't tried the foundation paints with an airbrush yet. As a potential solution to the problem, he suggested trying a non foundation paint that was close in colour to Iyanden Darksun. Luckily, GW does make a regular paint that is close in colour: Vomit Brown. I plan on trying it on a test model to see if it will result in a finished model that matches the ones I've already done.
The Painting Corps blog had an airbrushing week just a little while ago. I plan on checking out their series of articles to see if they have anything to say on the matter. Has anyone else encountered similar issues?
Also, my BattleFleet Gothic ships are in the process of being basecoated! The Battle Barge and two Strike Cruisers have been done so far. I'm not sure what to do with my third Strike Cruiser, since it's the Forge World Grey Knights cruiser. Obviously, it shouldn't be the same colour as the Imperial Fists' ships! Forge World has their model painted grey, like the Grey Knights armour and Thunderhawk. As chamber militant to an ordo of the Inquisition, however, it would also make sense to paint their ship black. I don't know of any background which states how the Grey Knights' ships are painted. Anyone know?