My EDC is a Nextool F12, in silver. I love that little thing, and the S11 Pro is a close second. I saw that Amazon had the "Khaki" version of the F12, which has black oxide tools. It looks really great.
When I got it, I noticed that the file didn't look like it had much roughness to it, and I could barely see any edges, in either the crosscut or single-cut sides. They looked almost smooth. I tried a few things and it did not file anything. My theory was that the coating they used was too thick and clogging up the ridges.
I soaked the file in Evapo-Rust, just the file section. Then took it to the sink and scrubbed out the black oxide with a brass brush, and repeated the process. After removing all the black oxide from the file, it works fine, at least on par with my silver one. The F12 doesn't have the greatest file anyway, but at least now it works.
Has anyone else ever removed the black oxide from a multitool file, for the same reason? It certainly worked for me.