All the articles I recall reading and the guides from Veritas and Lie-Neilson suggest using a diamond grit plate (or glass plate and sandpaper, or granite block and sandpaper). Can't say I recall reading anywhere about this, but I bought one when I was out of town visiting family and needed to flatten a combo waterstone I had left with my folks years before. (The state of the kitchen knives' cutting edges was not pretty).
I ordered this thing (from a popular 2 day shipping online merchant)... https://www.nortonabrasives.com/en-u...attening-stone
It worked so fast, I ordered one for myself back home and have not touched my DMT diamond lapping/flattening plate since.
Anyone else use this? What do you prefer?