Milliput is a two-part plumbers epoxy that comes in a fine-grained version and several colours; it is used by museums to repair china, by fine-detail model makers, luthiers, plumbers; the lad in my local hardware store says 'I never make anything without using it' and 'you can repair a submarine while it's in the water'.
It can be worked as it cures, and afterwards, and responds well to being molded with a wet tool for a very fine finish. I have posted here before about using it to make an ergonomic guitar pick.

product info
here