Here is a method I came up with for kneading medium grind kava. It has the advantages of being nonmessy, quick, and keeps your hands clean and dry. Kavalactones don't cling to your hands, thus you get less dermopathy on your hands. I used to get cracks in my skin near my fingernails when I kneaded by hand. Now I don't get dermopathy anymore. It is easy to knead a whole gallon of kava in seconds. All you need is a juice container, a potato masher, and a strainer. I prefer small pore paint strainers.
Blenders can also do a good job, but you still need a method to squeeze the kava from the medium grind. This method could be combined with blending or sonication for even better efficiency.