I thought it sounded too good to be true. As a filler, as long as it's encapsulated, it should work.

The fiberglass rope is made for this purpose. It has been used for fillets to install skeg on surfboards for many years. I'd imagine boatbuilders use it as well.

You can also pull fibers strands out from fiberglass cloth and make your own rope.

> It is doubtful that insulation or even rope has the proper surface
> treatment for adequate long term epoxy bonding.