The seam has to go but as Bob has said, do a good cleanup first to remove any residual mold release agent. I start with some liquid dish detergent like Dawn or Palmolive, wash it down 2-3 times. Then I use the wax and grease remover - and unless you are letting it sit for a few days, the water based stuff will evaporate better. If you are just going to clean it then go off to work on something else, I really prefer the solvent based W&G remover.

The gel gloss on a 'glass part has to be removed and a tooth for primer made. I tend to use a little finer wet and dry paper then many - 180 or 220, but it does provide a good surface. Then - I use epoxy primer, and the best that I have found is Southern Polyurethane. You really don't even need any sealer as it is fully sandable.

If you can find a Wescott's catalog, they do have a pretty good write up on finishing fiberglass