A smoothing plane is used to make a wood surface perfectly smooth. To do this you need to set your blade very fine, but even so the corners of the blade will leave an undesirable small mark on the surface.

To overcome this you must do something about the sharp corners. The way I do it is to form an oval shape curve on the corners as shown in the adjacent diagram.

The second photo shows how you lift one corner of the blade off the sharpening stone by about 3mm so you are only honing about 3mm of the edge on the other corner to form the desired shape as in photo one.   If you are worried about the blade digging in, only drag the blade over the stone and don't push the blade forward over the stone.    Get your basic shape on a 1000 grit stone then refine the shape with the 6000 grit stone.   This small extra part to your sharpening will prevent the blade leaving tracking marks on planed surfaces that are wider than your blade.

The last photo shows the resultant corner of the blade that has been relieved.