Can I flatten your stones the same way Id flatten normal waterstones (Silicon Carbide on glass), or is the lapping plate required?
Yes, you can flatten these stones with your method however, the lapping plate will get them flatter and you wont need another set up. The plate is a base that supports the stone while sharpening. You peel the cover back and flatten the stone periodically as you are sharpening. It holds the stone very securely.
