I saw Youtube videos on sharpening files and I wondered how they did it. https://www.youtube.com/results?search_query=sharpening files Some use acid (sulfuric acid or even vinegar) but how does this work ? When the teeth are blunt or dull, I thing it cannot be chemically sharpened. Admitted, I did not try it out yet, but I have strong doubts about it. What are your ideas on this ?