Oh, for the airline? A simple in-line water trap has worked for me. If you're really paranoid, stretch a piece of nylon (like pantyhose) over the hose end, where it would screw into the airbrush. It should catch anything that's made it that far.
I guess you could purchase one of those in-line, clear, after-market fuel filters and somehow work it into the air hose. There's probably a point of diminishing returns in there, somewhere, though.
-Fred