EPS blanks do not need to be sealed but sealing helps to use less resin and also provides a bright white uniform surface if you are doing color or artwork on the board.
You can simply squeegee lightweight spackle on the board (mixed with bottled water to a mayonaise consistency) in a thin later to fill the small holes between the beads, or mix up a paste of epoxy and Q-cell and squeegee that on (makes a stronger board).
There is also the option of finish sanding the board to 320 grit sanding screen to close up the cavities between the foam beads.