That's right, gelcoat doesn't pop because of rotted or soaked coring.
Easy to check your transom yourslef, don't need silly gimmick meters.
Do you have a drill and 1/8 inch drill bit and know how to use it and acess to inner side? Say yes and we wil provide the 1-2-3 steps to do, tic tac toe
I've had problems in a few places with gelcoat starting to lift and peel, never on hull, only on those molded covers like anchor locker cover and my cabin roof hatch, and my pulpit.
Had to abandon the pulpit - beyond restoration, not one shred of glass in there, only one layer of chopped strand, a ton of filler. I was taking out a slightly wet core (not plywood or balsa by the way) and I cracked the thing right thru, 1/2 way across, whoa, what a POS.
Using the pulpit as a mold now to make a proper replica, will be hitting the 1708 in mass, got to sharpen those fiskars!!! He He He. When I get done with it, won't need any core at all !! He He He. 1/4 inch solid glass skin, going to be a "No automobile polyester body filler crap zone" pulpit, he he he.
Getting back to the noncored covers, it looks to be like voids and gelcoat too thin, including the pulpit peel. You may have voids water penetrated and froze and swelled, etc, etc. Luv those voids, so many benefits of void construction.
Forgive me, at the end of my annual rope for temps too cold too long to do squat on boat but make plans and templates, and stare at all my new electronics and SS264W sitting in front of me. Any week soon, I need a 60 deg. day to keep faith, so help me !!