i have not tried whith an iron file. Thats quite a “brute force” tool for somthing as fine as a tiny knife. However i do imagine it to work a bit faster.
The problem however is that an opinel is convex. So grinding the primary bevil of, will not yield you a flat side.
I just ground the backside flat. and when the flat plane on the back side almost reached the cutting edge, i started sharpening the secondary bevil on the other side(the large shiny bevil, not the tiny hair line primary bevil) till they met.
My secret weapon was a lighter fixed with some 3m dubble sided automotive tape. (the gray gummy tape with the red plastic film to seperate layers in the roll) costs 1-2$ on ebay
If also used small wooden blockes wrapped in electrical tape in the past. Just taping somthing to the knife when sharpening a large surface really helps. Since the knife is so small and especialy a opinel nr 5 will want to fold while sharpening. Since it does not have a locking ring.
So i basicaly made a little handle to ease the sharpening. Keep paying attention though. Dubble sided tape can come loose. And it sticks a lot better when it’s on a polished surface. I almost did not get it off the flat side after sharpening the beviled side.
I also wrap the handle in parafilm. Just to keep water and sharpening gunk out.
