I have an old burnishing tool I got from an estate sale that works really well. It has an almost sharkskin texture to it, almost like it's been sandblasted by really fine grit.

As far as how big the bur should be, I prefer for it to just be able to catch on your nail, but not necessarily hold onto it.

I might have to give the diamond plates another shot, I'm guessing there's a big difference in quality between brands. I have some ceramic stones, but I hate getting my tools wet in any given situation.

And when I do hit a nail with my brand new plane (sob)

I've only done this once, and like an idiot I thought it was a knot and backed it up and ran at it again like 2 more times......