You shouldn't leave styrofoam in a hull anyway, it soaks up lots of water, increasing the weight of you boat over time.

http://www.quickform.net.au/
http://www.mouldcam.com/index.php

There's a similar company on the Gold Coast, can't remember name, that produce their moulds with a rough, deep cut into styrofoam then cover that with a thick epoxy filler for final cut, then paint and polish.

Here's the filler, on first page.
http://www.atlcomposites.com.au/files/news.news_2009/newsletter_12_-_autumn_09.pdf