They are annoying to sharpen, that's for sure. If you have waterstones you can round over one of the edges and use that for the concave portion of the edge. But it gets very easy to dig into the stone when the contact area is so small, so use edge trailing strokes or just be very careful. You can also get a dowel, maybe 1/2" diameter, and attach sandpaper to it. Clamp the dowel down.
Basically you just need an abrasive media with an edge or a radius. A radius is more forgiving than an edge, ideally the radius will be the largest one that can still get into the recurve.