I think anything that you'd put over the flexing fairing compound would then crack and flake off. Maybe rhinolining or some such material would suffice?
Years ago down at the WorkBoat Expo in New Orleans I visited the SPI booth and they had a coating, akin to rhinolining, only this was a high gloss smooth coating. Can't remember what they called it (although the saleswoman's name was Judy and was 5'9" with blonde hair and favored bourbon to rum) but after looking at their website quickly I think it has morphed into SP Liquid Membrane
Superior Products International, Inc.
No idea what you're trying to cover that may or may-not flex, just an idea.