Should you choose to use polystyrene foam, the extra expense of epoxy resins may be avoided by (after shaping) coating the plug with latex house paint, several coats. Them applying mould release wax and PVA
to the surface. This prevents polyester resins from attacking the styrene
foam. PITA to remove the polystyrene foam should some stick to the part (some will). Easily removed with acetone in a squirt bottle. The acetone will dissolve the foam quickly. Recently did this on three complicated fiberglass parts. PITA in all.
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