Serrated knife

[Mosby]

Speedy sharp works great for a variety of blades. About 11 bucks on Amazon.

They also work great as a striker for a ferrocerium rod fwiw. Dual purpose and throws a lot of sparks.

 

[Nosferatu]

I found the best way to sharpen a serrated knife is to grind the serrations off!

 

[woodswise]

I use a DMT serrated diafold sharpening rod. Comes in different grit. These are diamond sharpeners. Require no water or oil as lubricant, fold to secure the rod, last forever, almost. Serrated blades are more time consuming to sharpen, yet hold an edge when during rough work like joint separating.