I would hate to burst bubbles but I personally cannot believe that is an actual restoration and not a recreation. What's important to realize with a sword in a heavily excavated condition like that is that you're not just seeing a layer of rust on top but a complete decomposition and oxidation
through the metal. From a conservation standpoint there is literally nothing you can do. There is nothing left to work with
. Plus all of the pieces have essentially fused together making it impossible to access parts of the sword. Having handled pieces like this I can attest to how absolutely brittle they are. It would destroy the sword to even attempt to wipe at it let alone buff it. Hence, I am curious where you saw this. A link would be most welcome.
That said it is very cool! And even if it is a recreation it seems to really capture what this Katzbalger may have looked like in its working life.