Another option to consider is to surface-seal the rough concrete with a thin (1/8" +/-) film of 2-component, lower viscosity epoxy. Pour it onto the center of each stepping stone, allowing it to seek its own level just to the tops of the coarse aggregate--you may have to push it around a bit with the rubber scraper that you used to mix it. To keep the hardened epoxy from being too slick when wet, you can lightly broadcast some silica sand onto the epoxy while it's still a liquid. And to prevent the epoxy from running onto adjacent areas, make a mini-form around the outside of each stepping stone with a doubled layer of duct tape (with a light layer of Vaseline on the contact faces to act as a bond-breaker).
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