First read the information provided with the epoxy or contact the supplier to see their recommendations on whether and when you might need to SCUFF the seal coat to ensure a good bond of the fiberglass layer to the seal coat.

EVERY epoxy is different.

I use System Three epoxy and they tell me I can add a new coat without scuffing up the previous coating for up to 72 hours. MAS epoxy says you need to scuff the surface if it's no longer tacky.

That's a huge difference.

AND I emphasized the word scuff instead of sand, because you are just providing scratches to aid in a mechanical bond not sanding, the normal goal of which, is to level and smooth the surface. So, scuffing the seal coat is just a light pass over the surface with a foam block and 80 grit paper.

For more detail see, http://www.laughingloon.com/epoxy.html

Rob