I understand your troubles with the Tamiya white paint (thas why I quit using it some time ago) Yes, I'm fairly certain Tamiya does make a clear gloss overcoat.
They key to airbrushing Future is to use it straight from the bottle without any thinning and, above all, NEVER let it dry in your airbrush. As soon as you're done spraying it, load your airbrush with a 50-50 mix of isopropyl alcohol (available in any well stocked pharmacy) and hottest tap water. put this mix through your airbrush a few times and you should be alright.
If your brush still isn't spraying right after this, dismantle it and soak all the components, except any rubber seals, in straight isopropyl alcohol overnight and rinse them with hottest tap water afterward.
As a footnote, I highly recomend you invest in a good quality respirator mask with replaceable filter cartriges if you do not already have one. and since your spraying alcohol, spray in very well ventilated surroundings.
Its not as dangerous as it sounds, just use your common sense when you do this sort of thing and you shouldn't have any problems.
Good Luck!