Hi,

Just to make sure I'm not going mental here...... just bought some 'blue surfboard foam' started laying up with polyester resin and I got me a melty mess

I thought it was ok to use PE resin with PU foam ????

Or did I just get given some blue polystyrene??

WTF is going on?? Help!

HMMM, The only blue foam I know is extruded styrene. It's closed cell so doesn't suck water, but polyester does melt it. Sounds like what you've got.
And people are using it for surfboards, so selling it as "surfboard foam" is valid but misleading.
That's why accurate specification is preferable to generic vagaries.

Polyurethane foam usually comes as a "blown blank" in a roughly board shape.
Polystyrene is normally in a slab form.

I've made a couple of windsurfers from this foam and really like it, but a few years ago they increased it's density so it's now hard to get a light board from it.

If you stay with this foam and use epoxy, you'll probably find the board skin is a bit softer than you expect. It will dent fairly easy, but won't fracture so readily. Carbon rather than glass will give a much stronger finish, it won't dent as much, but a point impact will puncture it easier. If using carbon you probably don't need a stringer.