Do you have a worn spot or possible small hole in your kayak's hull? Try JB Weld's WaterWeld (approximately $7.00 at hardware and department stores). Yesterday when getting off the water, I noticed a worn spot on the stern of my keel. I have been wanting to try this quick setting epoxy, so I figured I would see how it works to build up the thickness of the hull in that spot.
WaterWeld is easy to use, just cut the amount desired from the putty stick and knead it thoroughly. Then just work it into the area you want to use it. It starts setting up within minutes, and even cures under water (at least that is what they advertise). When hard, you can even sand it smooth. It's also designed to bond to plastic, which makes it ideal for rotomolded and thermoformed kayak hulls.
I applied it to the keel in two thin layers, sanding between each. When it was built up to the level I wanted, I sanded it totally smooth and blended it to the level of the rest of the hull. It looks good, and gives me some extra hull thickness where the hull touched down when car-topping, launching, and sliding across rocks.
I am yet to see how well it bonded to the hull, given summer heat and the abuse I will put it through, but me initial impression is good. It will at least help me from wearing through the hull as quickly. Try it out...