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Hi Edbso,
Its common to use epoxy resin when bonding to polystyrene foam core materials. But that is not your issue. Also, its resin, not rosin. Rosin is a amber material used as a flux in soldering. Its also powdered and used by athletes and mountain climbers for extra traction/grip, as well - baseball pitchers, gymnists etc.
A better way might be to make a postive foam pattern, which you coat with epoxy resin, sand and apply 4 or more coats of beeswax too. Then push that into a liquid resin that is damned around 4 edges on a board. Use another flat board to get the back of the pattern flat. Check it with a straight edge. After the resin hardens you pull out the pattern. This makes a negative tool. Apply beeswax or other release agent to the tool, and do the fiberglass into it. Alternatively, you can model the part in 3D, and send it off to a CNC Router shop to mill out the negative tool cavity from soft wood. Then coat the wood with epoxy sand and polish.
You might also vacuum bag the part while it cures. Vacuum bagging is easy to do. All you need is a vacuum pump (like for HVAC), or a venturi pump and air compressor to generate the vacuum. Woodcraft sells supplies for vacuum bagging.
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