How not to...
I am repairing a board that has had a few nose jobs done, and I noticed a 'normal' filler like Plastibond or 2-part auto body bog on it when I bought it, so wondered how bad the inside was going to be. I have cut about 70mm off the nose end, and repaced it with a divinycell block. Anyway, I digress.
What polyster based fillers do to your board core:
Note that the polyster is melted even some distance from the filler (the dark coloured stuff), just from the fumes in the few minutes before it set.
Don't listen to Bunnings employees - use epoxy 
mark most people have some polystyrene packageing foam laying around , probabably a good idea to do a test run first, auto bog should have brought a red light
