Good video that shows what can lurk beneath a superficially solid grp transom. With my boat I used a diamond holesaw to cut a window in the inner laminate low down to assess the state of the plywood core. Based on that I decided to cut out the entire inner laminate and replace the plywood. Wish I'd had one of those oscillating multi tools for the job back then, mine was all done with a dremmel.

https://www.youtube.com/watch?v=FCzA-WF ... ture=share