Here's the two ready for spraying
As predicted the uncovered areas of polystyrene melted away, more rapidly and deeper than anticipated.
Now all that's left to do is let them dry, then glue the surface for protection, and they should be ready for painting :) .
Well I used very little glue for both of the boards, about 2 and a quarter tubs of sand and I may consider buying some more spray paint, but I definitely need some more sand.
Anyway I've started to glue the surfaces as a protective layer.