8 hours and no posts Lucky I have been working on Zombie apocalypse defense today
Tested out the files Practiced draw filing on the edge (I am more rusty than the Hatchet was before I started) Linseed Oiled the handle and will tidy up the edge tomorrow on the stones as now it is well past Beer O'Clock.....
How would you recommend sharpening the concave cutting edge on this billhook?
If you want to get rid of the rust patina on the flats of the blade, use a sanding block with 120 grit for the rough work, then wet-sand with 220 & 320 grit in a circular motion to buff it out. You'll still have some dark pits & spots; this is the "character" of the blade. Learn to love that "character" as you do the knife itself. Do this BEFORE you start sharpening the edge; it will make your work much easier and will help you cut the bevel more smoothly.
I would use a Dremel with either diamond cylinder or a 3/8"-1/2" 80-120 grit sanding drum and a chainsaw sharpening guide to do the heavy hogging out of metal required to get a proper bevel on that compound curve; this will probably need grinding on both faces. You have several deep nicks in the edge; given the usual applications for a billhook, I'd probably ignore them and NOT try to level them out. If you're a bit more careful in its use, those will disappear over the next 5-10 regular sharpenings or so.
After that, draw-filing with a large mill file (you've already done the bastard work with the Dremel) on the straight edges, then finish it all with the cylindrical/oval profile stones. You may be tempted to do the curved areas with a half-round file; DON'T. They're usually only available in a double-cut face (like a bastard file) and you really never want to use anything but a single-cut face for sharpening.
Get in the habit of touching it up with the stones every 5-10 uses for a while to establish a proper edge over time... this will yield the best edge while costing you the least irreplaceable metal on a tool that has important memories attached.
Cheers!
mnem
*final cut*