David80
I then applied TS79 (semi gloss clear) as the final coat, but it reacted with the AS11 in some areas, turning the finish an off white, faded look. (the AS30 was also affected, but fortunately is hardly noticeable).
I can think of a couple of possibilities.
1: You didn't shake the can enough. Especially with flat clearcoats, if the paint in the can has been sitting for a while and hasn't been thoroughly shaken, it can cause the paint which has been sprayed to carry an excessive amount of the flattening agent. This can result in a frosted appearance when the paint dries. Shake the can well, for at least 5 minutes. To improve the paint flow, you can place the can in warm (not too hot that you can't put your hand in it comfortably) water for a few minutes (and shake again) before spraying.
2: You applied the flat coat too heavily and there has been a build-up of the flattening agent. Apply light mist coats and build the depth gradually.
In either case, if the thickness of the paint hasn't obscured surface details, you may be able to fix the frosted appearance by applying another coat of clear gloss, then follow the steps above when you apply your flat coat.