[Madact]

Before you get too enthusiastic about fiberglassing: if you're using polyester fiberglass resin, it will eat the styrofoam.

For bonding to styrofoam you want to be using epoxy resin, and in particular one without the stronger solvents - generic 'marine grade' epoxy resin is generally OK, as they tend not to use the more volatile solvents that attack styrofoam (this is because solvents tend to leave an ever-so-slightly porous surface, which is bad news in marine applications).

If unsure, mix up a spoonful of resin and apply it to some scrap styrofoam. The styrofoam may just 'go away', which is obviously bad, but otherwise leave it on overnight to be sure, and test for adhesion the next day. A small amounts of solvent can be just as bad, e.g. car body filler directly on styrofoam will often look fine, but a day after application will have eaten about 1-2mm underneath, leaving the putty completely un-connected to the styrofoam.