Hello Yves,
This appears to be a very old budiak. The surface oxidation looks well stabilized, at least judging from the color and apparent texture of the surface. If there is no active red rust on the surface, I would be inclined just to oil it, wrap in a rag dampened with oil, and leave it as it is. If there is active rust, then gentle removal with sandpaper or steel wool would be a good idea, then oil it.
Attempts to remove the dark oxidation would require some effort. Since the surface is pitted, an acidic de-rusting solution or gel might be an option. However, it really depends on what your intentions are. If you just want to preserve an old artifact, then a conservative (less aggressive) approach is the way to go IMHO.
Others here may well have a different perspective or recommendations.
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