First I shimmed up any really large gaps with thin pieces of wood, epoxying them on. Then I routed all the edges.
Once I sanded down the bumps and the previous coat of fiberglass, there were still some larger structural gaps that I wanted to fill with thickened epoxy.
Some of my epoxy kicked off Before Its Time with billows of smoke. Exciting.
Then I sanded everything smooth as butter with an orbital sander. Fancy.