Its not GRP... So what is it...?

[Berton]

A 24-26ft Folkboat derivative, no idea of the builder/make/model/age, and had thought it was made from fibreglass (the inside of the single skin hull has a fibreglass layer over some strengthening elements) but since removing some old thru-hull fittings, it has become apparent that its not a fibreglass hull. The material is off-white, opaque, about 12-15mm thick, and when cut is solid throughout as if it were molded/cast.

Any ideas of what the material is and which process of filling/repairing the unused thru-hull apertures would be greatly received.

Thanks all.