If you're doing a clear, white board or pigmented lamination no need to fine sand. You can leave it at around 80 grit and glass. It will use a bit more resin as the small voids in the foam will fill with resin before the glass is saturated.
If painting or tinting the resin you can either spackle or fine sand up to 150 screen to close the voids in the foam. Spacking is easier...