Originally Posted by Miguel
Hello Ian, thank you very much for your advice, much appreciated, most likely prevented a catastrophe, white vinegar it is then. I just purchased a new quantity so should be OK
White vinegar is not likely to work (but it is very good for cleaning the white deposits on the kitchen sink and shower).
You should use either Nital (around 4%) or Ferric Chloride (around 20%).
Diluted Perma Blue will also work but you need to employ the "reversed" process: etch the whole blade to uniform black, then gently wipe the excess blackening with cotton swabs and Pre Lim, until you get the right degree of etching to reveal the watering.
Also please note the surface to be etched should be polished to 2000-3000 grit.
For more details see the link below: