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The hardness of steel can span the hardness of common natural minerals.
Depending on how the steel was annealed and tempered
R62+ can be too hard for quartz rich hones. Coticule hones
have...

JimR asked: "Why should he stick with synthetics? "

Only because that is what he has now and additional synthetics
can most reliably continue the progressions.

You are 100% correct that...

Plane blades are not exactly the same as razors I suspect you need to stick with synthetic stones or perhaps abrasive films on glass. My local woodworking shop has 0.3micron abrasive film for plane...