Grayboe and most upper end stocks are made of Fiberglass. They use fancy words…Solid, Homogenous, Fiberglass Epoxy Matrix! WOW! That sounds Awesome!, LOL! Which, in normal person terms mean: Stiff, Epoxy imbedded Fiberglass sheet that is all the same throughout, LOL!

And yes, any laminated composite like Fiberglass, Carbon Fiber, G10/FR4 & Micarta Phenolic all make a very fine almost powder material when sanded.