With my limited knowledge of the "Fantastic Plastic", I don't think the gelcoat is the moisture barrier for the fiberglass. What are you finding inside when removing the foam? As related to the hull laminate. I would think the hull laminate has been compromised and will require cloth and reglassing on both sides if possible.
I am sure a more experienced response is forthcoming.
A sandblaster is a harsh way to remove paint/gelcoat from fiberglass.
Good Luck.