: 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?

basic answer would be to stick w/ cloth or matte. batt insulation is mostly air and with random strands , so you'd have to compress it to achieve composite qualities otherwise its just a resin sponge on its own-so just use matte or cloth. If your mould has to be stiffened, you could use semisoaked batt as the core or for making coffers/beams between 2 skins of cloth or matte over/under that, but it really would be difficult to judge the stiffness without testing it to get experience first. could easily be just a waste of resin - not worth it.

if your plug is quite 3-d and to be kept, carefully reconsider using polyester as one of its big hits is that it shrinks so it unfortunately can grab inside plugs.(ok for boats/yaks as they pull away, tough for the mouldmaking) If parallel sides or tight convex, consider piecing the mold. seal styro foams or use urethane foam as the plug.

if your sculpture has lots of undercuts, consider using rtv as the primary mold to get the detail and pull off the setup/plug and then piecing a haphazard polyester one outside of that as the structural backup when casting.

-mick