The way they make "pewter" figurines, now-a-days, is to mix bronze or steel dust in with the resin during the cast, the weight of it sinks it to the exposed surface, then hit it with a buffer afterward and all the "outies" shine up nicely. The clearcoat as has been mentioned
Brake drum dust works well, just ask the guy/friend to give you the dust off the "top" of the pile, that hasn't been marinated in machining coolant.
A litttle chopped strand mat helps with strength, but the trick is to keep it away from the exposed surface soo that the casting doesn't appear "hairy".
Nice job, thanks for documenting so well. Resin formulators will sell casting resin off the shelf, it'll probably be much cheaper than TAP, but might require a larger minimum buy.
(I teach fiberglass classes at a local Jr College, and this is one of the class projects - though not in my class - done each semester)