Future is great stuff - it creates beautiful clearcoats - but if you airbrush it, don't thin it at all. Spray a couple of light dust coats and let them dry - don't try to build up a gloss all at once or it's not gonna be pretty...............
Gradually work up to a full wet coat, leave it for 24 hours, and as glusnfr(?) said, the stuff is self levelling.
Clean up can be with ammonia and water, or isopropyl alcohol, but I just use automotive acrylic thinner - it removes everything and smells very bad, but I use a spray booth.
Nice thing about Future is it smells nice - kinda fruity. When you can't smell this odor on your model, it's pretty well dry - if you can smell it, you'll probably damage the finish during handling.
Cheers, Lee Tree