Stringers

[Curapa]

I have never heard of a pro recommending to leave rotten plywood in a boat and just cover it up. That being said there are boats with foam filled glass stringers that work fine but I have seen none that were smaller than 3-4" wide. Skip the pourable methods, they are heavy and expensive. Pull the cap or cut out and glass back in what's need to gain complete access. Carefully cut out a stringer with a sawzaw, use it as template for the new ones. A grinder with a flap wheel is your friend when working with glass.