Use a round rod or the narrow edge of a stone, never a wide flat stone. A recurve has a concave section that a flat stone physically cannot reach, and repeated attempts flatten the curve out until the profile is destroyed.
A recurve curves inward toward the handle before sweeping out to the tip. The inward section is concave, and a flat surface can only touch the two high points on either side of it.
Keep going on a flat stone and you grind those high points down until the concave section disappears. The knife ends up straight-edged and much shorter.
A round ceramic or diamond rod, the narrow edge of a stone, or a slack belt. All three can follow the curve. Rod-based systems and slack-belt grinders handle recurves well; fixed-angle systems with wide flat stones generally do not.
Work the concave section with the rod and the convex belly however you normally would.
Make sure you own something that can sharpen it, or the knife will quietly stop being carried. That is the honest practical warning about the shape.
Hawkbills have the same problem in a more extreme form. See hawkbill.