Pictures will definitely help. I interpret from what you have written that the problem is a leak at the centerboard trunk due to rotten wood. I am not so clear about the presence of fiberglass and resin behind the wood. My guess is that this was used as a patch when the leak initially showed up.
You will want to remove ALL rotten wood. The best repair method would entail replacing the section of the trunk that incorporated the dry rot. Fiberglass and resin will not adhere to bad wood. If you have moderate woodworking skills, this should not be a huge job. When we see the pictures, the group will have a better idea of what you're facing and will probably have some great suggestions.