At least I think its oxidation.  White film that develops when finish is 4~5 months old.  I have had this happen using both Brodak and Sig lite-coat.  Polishes off easy enough, but I wondering what causes it or what I might do to prevent it.  I usually have to spray in humid conditions so I generally use retarder.  Could that have anything to do with it ?

Brodak and Sig,  I stayed with the brand's thinner, and retarder.  No mixing brands, even though I have posted questions about that.  I was just curious.

When you sand and polish the surface, you break the "skin" of the dope, opening the pore up. This allows contaminants and oxidizers in. If you keep a coat of wax on it, that limits the intrusion, but ultimately, it will oxidize slightly. As you note, it's pretty easy to bring the shine back. It's worse, in some ways, with catalyzed polyurethanes unless you don't sand them. Once the skin is cut, you open the finish to this sort of thing. Hey, nothings perfect.