I tried 3 different colored markers as resists, black, blue and pink. all three turned out very well. Here are 2 of them below.
I used sticker label paper with fancy designed cut out on them. They turned out pretty good ( though the paper started to disintegrate, so when using this as a resist I don't think I would leave it in the acid more than 50 minutes) I used the paper resists on Vintaj blanks, so I got this kewl effect of dark ( the vintaj finish) where the resist is... yummy! and I cannot wait to play more with this idea.
I used stamps and stamp pad ink on 3 and only one turned out well. The other two the ink must have come off and it was more or less just etched. The one that turned out though is wonderful!! Now I need to figure out why it worked and the other 2 didn't. It's possible I had the other 2 in the acid too long. I was experimenting with times ( 20 to 50 minutes) . the 20 minutes just wasn't really long enough for any of the batches.. 40-50 minutes seems optimum. I would suspect the thickness of the metal might play a part also... and how well the ink stuck to the metal..