You' re right, they don' t feel that sharp but they are. You can use just about any flat sharpener as song as it is wider than the heads. I reccomend using a coarse stone first just to get all the edges the same. Just place the blades flat on the stone and push the head foward. Very easy to get them sharp.

I use a Smiths tripple stone (coarse, medium, fine) and they come out very sharp. They don' t feel that sharp but they slice through stuff just as good or better than the replaceable blade heads. Be careful with that tip.