I tried using apple cider vinegar to force a patina on my Composite Leek. The results were.... unexpected. After two hours of soaking, the D2 edge had taken on a nice dark patina, but there was a great deal of marring on the (normally) stain-resistant Sandvik spine. So I figured everything would look better in time...... and left the blade in for about 10 hours. The results weren't perfect, but were quite pretty.
