As I said before, cross-linked polyethylene is a bit different that linear poly that a kayak is made of. You may be able to get a little bonding, but the chemical properties are different. "Cross-linking" is a chemical process that happens during the building process. While cooking in the mold. I am told that once the material has fully "cross-linked" it is done. If the boat is under cooked, you may be able to get a slight weld bond, but the standard answer is no. You can not weld cross linked polyethylene. Now, mechanical bond maybe. Just not the kind of structural weld you would need or one that is possible on a kayak.

Plexus? Good stuff, but doesn't work well on the cross linked type of material either. The thermal plastics they use it on are normally thermoformed rather than roto molded. Different plastic.