I have recently researched sharpening knifes, skews and cutters. I have credit card and several diamond plates. To lengthen the life of them use their lapping fluid. It's ridiculously expensive but is far thinner than oil, and still has the ability to lubricate. I started researching this because I had a 1200 EZ LAP diamond plate that had lost a fair amount of its grit, even though it was only 8 months old. I had been using oil, and the "slurry" generated had adhered itself into the grit. I was able to clean it with a number of products and got some of it's grit exposed again, but it is still not what it was before.
I had never sharpened any cutters, but I had saved them, throwing them into a corner of a drawer. There was quite a few there, I sharpened them all, and tested some of them. As mentioned, I found they were improved but not like new one. I guess it depends on what you turn and how "thrifty" you are. They seem to cut wood well, and acrylic ok. I watched the Woodcraft video where they said to use a 600-1000 card and just rub it around 10 to 15 times.....I found it was far improved to follow as previously suggested, and rub it around 50 times or so each direction. I would go 300-600-1200 grits. I did not start at 300 and some of mine where "well used"....I might go back and try that.
I also have been able to get some of my knives to a razor finish by ending with a leather strop and compound sticks...anyone have any idea it that would help the carbide insert?