well actually Klear makes the clear parts look better! a lot of us dip our clear pieces in future (Klear for you all) and let cure 48 hours. Makes them look less ;'thick' and makes them nice and glossy!!! (VERY glossy!)
Here is a very well done article about using Future (Klear)
http://www.swannysmodels.com/TheCompleteFuture.html
As far as matte varnish, I think it would do just that... make it matte. Some have said Future will make it shiny again but I have never experimented with that to know. (I feel an experiment tonight, good question)
As far as cleaning the airbrush I just spray an ammonia window cleaner through mine just like cleaning with any other thinner, then follow with clean water... You will see it in the web site article, but windex (i.e. ammonia window cleaner) will dissolve Future so if a piece comes out not looking just right you can take it right off!
Enjoy the article!!! It's very well written and gives ya just what you were wanting to know and more!!