Paul and Mike as usual are right on. Here are a couple other ideas. I made several molds of architectural gingerbread out of the silicone RTV product shown below, and they worked very well. TAP Plastics also has a urethane mold material, but I haven't used and therefore cannot recommend it.

: This board was a wealth of information for me when i was building my kayak,
: now I'm hoping the resident fiberglass/resin experts can help me with a
: slighty OT project. I want to make a fiberglass mold of an original
: sculpture I've carved out of foam. My plan is to seal the prototype, coat
: it with a mold release and then use polyester (cheaper than epoxy) resin
: impregnated fiberglass to build up the mold shell. My question is if I
: could use unfaced fiberglass insulation (maybe 2" thick), satured
: with resin, to build up the shell quickly and cheaply? Or do you think i
: should stick with multiple layers of good old 6 oz. cloth?

: Thanks,
: Jim