The single most frustrating aspect of making reeds is sanding them flat after the gouging stage. One trick I’ve found is to sand before shaping so you have a larger base to work with and the cane doesn’t ‘rock’ too much creating and uneven surface. But the biggest problem has always been the sandpaper itself.
This is why I’ve finally purchased a DMT Diamond sharpening stoneto use instead of sandpaper. It’s large enough(10×4 inches), has a coarse and extra coarse side(equivalent to 220 and 325 grit) and best of all- it’s flat and will always stay flat.
So far it seems to be working quite well. I start on the extra coarse side and then finish on the coarse side. I’ve made five reeds with it, and I’m achieving flatter reeds faster.