There was a whole lot of experimentation back in 2017-2018 here on the forum. we tried, mustard, vinegar, and all kinds of crazy stuff. when my wet paper towel dried up and blew off, I realized that it’s all mostly unnecessary 
of course, it will depend on the metal. it appears that the carbon content might play a big role.
search for “metal engraving” or “metal etching” here on the forum and enjoy the rabbit hole! 
here’s one from another thread that has the stainless bottle opener example: Etch Steel with... apparently you only need a wet paper towel. No mustard or vinegar. lol
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