If it is the spray can dull coat: Testors dull coat is DULL and I mean DULL! When there is too much applied, it will give that frosty, flakey look. The amount applied is impossible to regulate so you will pretty much get it everytime. I would recommend the Krylon brand matte clear coat. You can get it at any art store. It gives a much better matte coat, almost a satin finish. But beware, too much applied will not frost but "buckle" the paint. Apply quick, light coats.
If it is acrylic dull coat: Same issue....it's very DULL. It too will frost if too much is applied . I would recommend cutting it with semi gloss clear coat and airbrush it on. I use 2 parts semi gloss and one part flat clear. This will give you a true clear coat and by adding the semi gloss, you will avoid the"haze". Hope this helps....
Andy