Sometimes people send us clipper blades which they have tried to sharpen themselves and this is never a good thing.
DIY sharpening on a flat (ish) surface removes more metal at the edges so the surface is rounded down toward the edges (convex).
The harder you try to sharpen your blade on a flat surface then the more rounded (convex) you will make it.
Sandpaper quickly makes them very rounded and a few light passes on a diamond stone is not so bad.
When your blade is rounded down at the edges we must grind the whole surface down to make it flat.
The blade pictured has been sharpened on sandpaper by Geoff. – The darker area in the middles was easy to make flat, but the lighter part towards the edge is rounded down. – This clipper blade would need a lot of grinding to get it flat right to the tips of the blade.