https://www.austinkayak.com/Weld-Rod-St ... 5512P.html. If you do plastic welding on a single color kayak with plastic welding rods which are the same color as the kayak, the welding can be pretty inobvious. Plastic welding is more likely to be visible on a camo kayak, and you may want to buy two different colors of welding rods in a effort to hide the welding.
You can buy inexpensive plastic welders which are basically just a soldering gun with a small flat plat on its tip. I recommend that you spend the extra money for a Hobie Welder Pro plastic welder. See
https://www.austinkayak.com/KC-Welder-P ... 1587P.html. These have a hole in their tip which you stick the plastic welding rods into. This allows you to melt the hull of your kayak and the welding rod at the same time, and lets you build up layers of plastic which are solidly welded to the hull of your kayak.