I use the same home-made tool for sharpening as I do for leveling frets on a guitar neck. Get a piece of 1/4" plate glass from the hardware store that's about 3-4" wide and maybe 13-14" long. Glue it with contact cement to a length of particle board. You now have a dead flat surface for your silicon carbide paper used for wet sharpening your blades, or leveling some surface, in case you put a bunch of frets on your kayak.