: Btw, surfboard do
: not use styrofoam.

: Bill H.

Actually, these days many do. Clarkfoam ran into trouble with the EPA so the traditional surfboard foam is rare now.

Windsurf boards definitely use eps. It comes in a multitude of densities - The variety used as a substrate for plaster finishes on buildings is quite dense (check your local plaster supplies dealer). I can step on it with all my weight on one foot (80kgish) and not leave an impression. If I jump, I can dent it.

There's also xps - extruded polystyrene. Commonly used as an insulating layer under the concrete in multilevel carparking buildings. It's a bit more expensive, but doesn't have the gaps between the beads to worry about filling before glassing.

You probably already know that you can use only epoxy resins with styrene foam of any description.. unless you want a really gooey mess!