I tested (for submarine use) Testors Dullcote (#1260) by submerging a painted piece of plastic in the water for a few hours. The clear coat seemed to work (held up just fine under the water), except as the piece dried after a few minutes, some tiny white spots formed on a small patch of the plastic, about the size of my thumb. This also happened slightly with some Krylon clear coat and even more so Testors' Model Masters Lusterless Flat Lacquer (#1960).
What causes these white spots? They seem to occur only in certain areas then spread from there. When I painted a submarine a few years ago, I suffered the same problems, even though I was using bathwater from a different place. Ironically, I ended up saying "screw it" and used the Testors Dull Cote anyways, and I never got white spots except in one small area and I never could figure out why the white spots formed there and nowhere else, despite running the sub in water many times. The white spots cannot be removed. I tried scratching off the white spots with a finger nail, and it's as if the white spots form just below the surface, either within the clear coat layer itself, or b/w the clear coat and the suface the clear coat was applied to.
My theory was that the "weakest" bond between the clearcoat and the Future/paint underneath allowed white spots to form first. This might explain the thumbsized spot on my #1260 test piece. Perhaps some oil/contaminants from my skin prevented perfect adhesion of the Dull Cote to the Future. But on my other two samples, the white spots seemed to form in random areas and in random patterns.
Since this phenomenon has occurred more than once, using more than one type of clear coat (Krylon and Testors) and when spraying the clear coat onto different surfaces (Future and Testors regular enamel), I have to think there's a common cause, but I just don't know what it is. Any ideas?