Hi, folks.
Back in the days when I could get Testor's Dullcote, that was all I used (spray can). The only trouble I ever had was I was never satisfied with the level of flatness, no matter what I did.
These days I use nothing but Gunze water-based acrylic clear flat, with no problems at all, and with a great level of flatness. Dead flat, actually. I've seen nothing flatter! I airbrush it on in light coats, and have never had any problems with unwanted "whitening."
Gunze also makes a lacquer-based clear flat, which cannot be used on top of anything other than lacquer paint (or so it says on the WARNINGS section of the label). It will definitely cause a dust-like whitish phenomenon, which I actually used to my advantage on an enamel-painted Sherman once. Pretty good dust effect.
I can't recall the product numbers for these Gunze paints, but the main difference is one is a water-based acrylic and one is lacquer.
Perhaps it is the lacquer that is giving you trouble.
Or perhaps you applied the clear coat too thickly? I have heard that can be a cause ot top-coat tribulations.