Tamiya Flat Base Troubles

This is typical of what happens when too much Flat Base is added, or if it hasn't been mixed. The stuff is pretty potent, so it may pay to be more dilligent with your ratio. Swanny has an interesting article on his site that lists mixing ratios for Flat Base & Future, but it should carry over to paint;

http://www.swannysmodels.com/TheCompleteFuture.html

I'm not certain, but I can hazard a guess.

When I was finishing up my Dauntless last week, I sprayed some Polly flat and had it spit white flecks all over the fuselages and upper wing surfaces.

Butz, I think it was, had the same problem with the same stuff awhile back, and turns out the white flecks come from too much flattening agent. I can't help but wonder if the white powdery stuff you're experiencing is similar. Or maybe that + a difference in factors. Spraying from farther away/wider nozzle/higher PSI etc? 

When using X-21 Flat Base, you must pay close attention to  your mix ratios, especially when using it with Clear Gloss (X-22 or Future). Any less than about 4 parts of X-22 (or Future) to one part of Flat Base and it will often give you that "frosted" appearance. Often, if the "frosting" is mild, one can fix it by spraying a coat of clear over it.

If you were able to wipe/wash it off, it sounds like you had an awful lot of Flat base and very little Clear.

I had a simmilar experience not long ago. I used Tamya dullcoat and ended up with a flat white disaster. I sought the advice of a friend and his advice work well. "Let it dry overnight, then re-shoot with the same dullcoat and be sure to do this in an invironment free of high humidity. Sure enough, the second shot of dullcoat completely eradicated the "frosty" appearance.