It's of paramount importance that the Sharpie be new. A new Sharpie lays down a thick coat of ink on polished steel and it doesn't leach along fine grind scratches the way it would on say a handsanded blade. Andy thin spots in the Sharpie ink should be addressed before etching.
I don't know what ammonium persulphate is, so I cant speak intelligently about its suitability for this procedure, but I'm glad that you enjoyed at least some success with it. With appropriate adjustments, to your technique, perhaps this will be a viable process for you.
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