why does nitro cellulose lacquer turn white in high humidty?
Bronzzy

It is called "blush" and it is moisture captured under the film before
it has a chance to evaoprate. There are treatments you can add
(retarder) and techniques to (sometimes) remove it after it's occured.

You can get "retarded" at professional finish supply houses (like Wood
Finish Supply). Here is a link to a nice write up on the subject.

http://www.finishsupply.com/starRetarderAero.html