While diamond hones will do a great job of quickly removing material they are notoriously unflat, and thus in my opinion not acceptable. I have a surface plate, and while it is flat, I don’t like the idea of constantly having sandpaper glued to it. Additionally a 70 pound surface plate isn’t very portable. What I finally decided on (shown below) was a hybrid approach.

I took a 1/2″ thick chunk of aluminum plate, and cut it to size (slightly larger than my stones). I then lapped if flat on both sides, using sandpaper taped to my surface plate. Yes I said taped, when you lap a surface this size, you don’t really need to worry about the paper lifting up and rounding over the edges like you do when honing a plane blade or chisel. It’s actually beneficial if the edges are a little rounded over (1/64″ or so), as this will prevent you from slicing open your fingers or hands on the sharp edges. Finally I used spray adhesive to adhere 220 grit sandpaper to each side. PSA sandpaper would be better, but no one in town carries it.

What I have now is a dead flat surface that I can use to flatten my waterstones. When the sandpaper looses it’s grit, I can pull it off and apply a new piece. Additionally, I could use this as an extra course honing stone, if I need to re-establish a bevel, or take a nick out of a plane blade.