So it did seep a little, and the resin made contact with the foam on the side and bottom edges, which I didn't seal in the same way as the sand.
Seal everything with PVA. Then seal it some more. ALL THE SEALING! The exothermic reaction must have got really hot, as it's melted the XPS foam in places where the heat got to it.
So today will be filling holes, finishing the shieldmaiden, and maybe the cliff. Still, success!