What is the best filler(to fill the seams) for fiberglass?

Model Magic, from Carl Goldberg. Comes in a white container. One applied and dried, it sticks and is easily sandable.

FF,
You need to use the auto-body "filler" and then a "glaze putty" if I remember correctly from my car body days. I can't recommend a brand since I am not on the up and up anymore it but if you go to almost any auto-paint shop, auto-parts store, etc. they can hook you up.

Just be sure to score the gel coat so the filler has something to bite into when it sets. Then follow with some glaze putty to fill in any sanding marks. Should come out perfect!