Thank you for the video, it is very good to see knives being restored once again. I have an usuba which was left in storage for a few years. Unfortunately the rust has eaten into the core shirogami. The edge crumbles easily during usage. I grinded it down by hand on a 240 grit whetstone. Did a new shinogi and edge, however after thinning the blade more rust appeared after the kasumi steel was removed during thinning. I stop and had to keep the knife away and no longer use it. I believe the core steel has rusted badly but the soft steel above was not affected much which did not indicate how badly damaged it was.
I really enjoy watching your videos. Sadly l cannot read Japanese, and so l particularly enjoyed the one l saw that had English translations of the commentary. But they are all very well done. Beautiful work!