After working the blank to the desired shape, the next step was to seal the blank by filling in all the holes. I especially had a bunch of holes from filling the existing fin holes and holes I ripped into the foam from creating the concave in my board (sorry I forgot to take photos during this stage).
In my case, I went to the hardware store and bought a small bucket of spackling, but drywall and filler would probably have been fine. I made sure to avoid anything oil-based that might cause poor adhesion with epoxy for the glassing stage.
I also used a scraper tool to work and spread the spackling.
Filling the holes
First I sealed the existing fin holes by shaving some polystyrene foam into little cylinders that fit snugly inside the holes. I then used polyurethane glue to fix them in place.
Using the scraper tool, I spread the spackling onto the board and tried to fill all the tiny holes I could find. It was better to overfill the holes as some fillers shrink. After the spackling was left to dry, I followed up with some light sanding to even up the surface before glassing.
Sealing the blank gave a much smoother surface and allowed me to “fix” any mistakes and holes I introduced during shaping, such as the ones in my previous post. Another reason that you might want to seal the blank is for weight reduction. By sealing the foam with filler (lighter than epoxy), less epoxy from glassing will be absorbed into the cracks in the foam blank. Though you might start to question if the strength of the fiberglass job will be as good as if you didn’t seal the blank. Maybe someone can let me know.