Bypass sheers or pruners can be sharpened in a variety of ways. To sharpen you only need to address on the upper arm which has a single bevel and flat back. The lower arm is left alone as it offers a ridged 90° surface where the "bypass" action takes place.
Similar to scissors, pruners are self steeling and best practice is to a diamond plate or a powered sharpener. For diamond plates you can use the the ones found on the Guided Sharpening System or Guided Field Sharpener. Match the angle of the bevel with the diamond plate and make several passes to restore the cutting edge.
For a powered option the original Work Sharp Knife and Tool Sharpener or Ken Onion edition Knife and Tool Sharpener are great options. Disassemble the sheers and again, only sharpen the upper arm. Match the angle of the bevel on the belt and draw the cutting edge across the belt, use light pressure and stop when the tip of the sheer is in the middle of the belt. Once a burr is created, reassemble and snip the sheers together to "steel" the edge and remove the burr.
