Regarding using HNO3 (nitric acid) to sharpen cutting tools, it
does work. The acid removes metal on the entire surface and the
edges become sharper again, at least temporarily. The same
technique can be used to give steel burrs a longer life too. One
has to weigh the inconvenience (and hazard) of using a powerful
acid, against the partial and temporary reinvigoration of
cutting tools.
Be safe and smart when working with acids. Learn how before you
touch them.
Alan Revere