I can not tell you I'm 100% sure, because I havent tested it yet but you dont have to gloss your PU you've casted... just sand it smooth till 2000 grid or more and polish If you want a nice mouldfinish. Add some layers of release, and use tooling gelcoat and unimould.
That way you should get a nice mould for prepreg...
Tip:
Mix a small batch of your casting resin, and try the release and unimould on it.
Keep in mind:
you will have to postcure your PU before making a mould, because you will have to postcure your mould with your part in it (you don't have to, but its better, it will prevent your mould to change shape will postcuring before prepreg)