Seems like with polyester resin, one needs to mix some wax with the final coat
so air cannot get to the laminating coats and they will harden fully.

Does the same thing hold with epoxy? If not a finish coat, is a layer of paint
needed to make it harden fully?

My agenda is I'm covering a seam. I could finish-coat it if such a thing is
available using epoxy... but I don't want to paint it because I want the option
of easily checking what's going on underneath.

If UV degradation is an issue, my intent would be to cover the area with wide
electrical tape - or some other tape that is easily removed/replaced.
--
PeteCresswell