|My first question would be: why are you planning to unscrew the toerail? What problem are you actually trying to solve?
The sealant which C&C used on their patented hull-deck joint was grey butyl tape. Generally a layer between the in-turned hull flange and the vinyl rub rail, then a layer between the rub-rail and the deck, then a final layer between the deck and the toerail. A diagram of the joint is included in the Owner's manual. I can send you a copy if you wish.
Not that I wish to diss Gerard, but for the past 35 years, butyl tape has been my go-to sealant. Any time I have tried some other new "wonder" adhesive/sealant, it has failed after 2 or 3 seasons and leaked. Anything which I have bedded with butyl rubber has never leaked. Your thru-hulls below the waterline were bedded with butyl rubber by the builder.
I would also be dubious about using a Methacrylate Structural Adhesive for a number of reasons: first, I am not sure how well it would bond aluminum to fiberglass; second, I doubt that it would cope well with the differential expansion of dissimilar materials in hot sunlight. This is the reason that so many late-model C&C's developed cracks around the acrylic windows which were permanently bonded to the fiberglass of the cabin-top (Mine included). When the differential expansion caused the bond to fail and my windows to leak, I took them out, cleaned up the framing recess and resealed them with black butyl tape and bolts. They have not leaked a drop since. Also, if you do manage to permanently bond the toerail with Methacrylate Structural Adhesive, this may greatly complicate any future repairs in the event of a collision or docking damage. Further, the permanent flexibility of the butyl in the joint allows the joint to work and shift, as the hull changes shape to adapt to waves, haulouts, launches, etc. Your hull and deck are not a perfectly rigid structure.
Butyl has kept my boat dry for 40 years. In the event of a dribble along the toerail, I simply tighten up the relevant bolts and the leak stops. So that's my two cents worth on the topic.
Marcus from Carriden
Mk III, Hull #847