hull prep question
It sounds like you are grinding through a number of materials. If the grey material is comparatively thin, like a thick coat of paint then someone has applied either a vinylester or epoxy barrier coat on the boat during its lifespan. There were also grey polyester fillers that a lot of boat builders used to fair between the sump and the ballast keel.
More than likely the blue fibers in clear matrix is a resin rich polyester laminate which is very typical of that era. Unless the boat has had a peel and has been built back up, it is unlikely that there is epoxy anywhere in the boat''s construction.
1971 is just before the period of the worst blister prone resins. That said, the resins of that era were typically mixed with a lot of accelerator which can result in pourosity over time. In otherwords a barrier coat would be good idea.