Another idea for sealing the foam is using wood glue before the resin. I did that for one of my projects; after I carved the foam I coated it with a few layers of wood glue (I let each coat dry before I put on the next), and once the final coat was dry I spread on the resin. The wood glue will stay slightly tacky so the resin will stick to it, and it usually can help give things a flatter, smoother surface.
(This is just foam and a little bit of craft foam on one side to give it a slightly thicker surface there.)
(I did use a little bit of PaperClay so I could more accurately carve lines and to easily round the edges. Then it was all covered in wood glue, which is what makes it look yellow. After that I spread on the resin.)
(And here it is painted and finished.)