Thanks for all the answers, guys. 

I have had Testors clearcoats blush.  That is the white appearance due to high humidity.  Lacquer thinners evaporate so fast they actually cool surface. If the dewpoint is too high- close to ambient temperature, it condenses on the paint and appears as a white fog.  Usually this is summer time really damp weather.  Indoor humidity in winter is usually ver low. It is the humidity where you are painting that counts.  But in summer on warm days, try to watch out for really muggy or wet days and avoid lacquer glosscoats.

You may have applied the MM flat laquer too heavy.  It comes out much faster than paint in a rattle can.  Light mist coats are recommended.  

There is no problem whatsoever applying this over enamels.  I have done that with over 200 models.  MM enamel airbrushed then  MM flat from the rattle can. 

*You must let the enamel  (or any paint for that matter) dry for at least 48 hours if not for several days. Then flat in mist coats then a slightly wet coat.

Always let your paint cure.

Proper mixing is VERY important, not just a quick shake or a stir but thorough mixing at least a minute or two.

I spray flat clear lacquer over enamels and acrylics all the time with  no issues . The first couple of coats need to be light so they dry fast reducing any type of reaction on the acrylic or enamel. I let the acrylic and enamel cure for at least a couple of days before spraying over the lacquer dull coat.

How long did the enamel cure before appling the Testors dull coat? 

There's a whole host of things it could be. Since you got better results after shaking it probably wasn't mixed well. 

But yes what the guys listed can all be involved here: 

Humidity

An old can that's gone bad

Too heavy a coat over acrylic 

Not mixed well as you said

I'm not one to bet on the results of lacquer on top of enamel because I've seen it work and I've seen it go gittiwonkers too. Usually it's not hazing though but rather crazing  or wrinkling and it may not be right away either, though it could be.. Wrinkling could be pretty immediate, crazing could be in a year or two or more. Not saying it will, just saying I've seen it and more than once.

Guess it's not humidity then,

I use varnish and lacquer over enamel all the time without problems.....unless it's humid of course and we get heaps of it here in Oz .

Mick.

Snibs

Is that a varnish? humidity can cause that.

 

No such thing as humidity in winter - especially in the Great White North country...

Depends on where you sprayed. Outdoors or unheated garage perhaps? I never spray in cold environments.

Snibs

Is that a varnish? humidity can cause that.

 

No, it’s MM lacquer flat coat, and it’s cold and dry up here in the North country right now.

Hmmm, lacquer over enamels. That might be part of the problem but this is the first time it’s ever happened.

Is that a varnish? humidity can cause that.

Well let's hope that's all it was, then.

Good to hear!

Sounds like a plan to keep in mind for the future but I just tried it again with the can getting a good shaking and it turned out a lot better. 

My guess is can was too far away, paint dried before it got to the surface.

I've done this a few times with various flat clear coats using an airbrush.

Years ago Stikpusher taught me a neat trick. Lay down a wet coat of gloss over the frosting, it should clear it up and when dry you can try again with the flat. Stik's trick has saved me more than once. Seems counterintuitive to lay down a gloss coat over flat, but it works.