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Knife sharpening does not require nearly as fine stone to get great results.
Going higher then 6k is really a waste of time because the edge will be gone at the moment you hit the board.

So how are you going to be able to tell if the stone is good for razors when you use it on knives?
Magnification will not tell you anything about the shave you are going to get.

IMHO you get better feel of the stone if you do not strop.

you do not have to go down to 4k, just run the razor on the 12k and switch stones.
You can also try one of the stones after the other and see what happens,

The nakayma is not as uniform as the Shobudani, the pic does not show the actual features.
If the stones indeed are equal in performance I'd go with the more uniform one because uniform stones of...