i ground it back to the ply and left an edge to seal to which i faired in.
i then used Marine Epoxy - Duncan Yacht Chandlers
to cover/fill up the ply to get it watertight again.
letting it cure for a few days since boat is outside so will finally sand the epoxy smooth and apply some white flowcoat to finish the job and should be good to go.
thanks for help folks-will put up more pics of completed job soon.