through the hull. If you hit an old rebar spike, it might happen. You really don't need to fill the gouges.
If you do hole or tear the hull, there are plastic welder specialists. Ask your dealer or experienced paddlers.
One could try to fill a gouge with 3M two part polyurethane adhesive, or with West G-flex epoxy. But you won't really accomplish much by doing so.
Heel and Pegpads™