Looking for a product that I can apply to plastic, that can harden, and that will be water proof.?

I’m making outriggers for my kayak. I’m using pvc, large ones with 5 in. diameters for the buoys, them I’m filling them with foam. Most people on the front of them will get a block of wood and tapper it to a nose and slide it into the pvc pipe. I don’t have those tools so I thought of using the top of 2 liters... show more I’m making outriggers for my kayak. I’m using pvc, large ones with 5 in. diameters for the buoys, them I’m filling them with foam. Most people on the front of them will get a block of wood and tapper it to a nose and slide it into the pvc pipe. I don’t have those tools so I thought of using the top of 2 liters instead, but id like to coat them with a hardening material. I don’t know what that would be.
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