Originally Posted by rossnzwpi
Robert Q. Riley describes using polyurethane foam - which has a very fine grain and is not melted by standard polyester fibreglass resin. Polyurethane foam used to be readily available in insulation supply companies - I'm not sure if it still is.
Polystyrene foam is much more coarse and doesn't cut or sand in the same way. You can use a different, more expensive fibreglass resin (epoxy) to avoid melting polystyrene.
Just use polyisocyanurate board (trade call it PIR) Cellotex or similar - just peel off the foil off one side and paper off t'other (it really stinks when you do it!)
Works well with polyester resin. Also builders 'squirty foam' is useful and can be used to fill gaps or join boards.