This method uses electrolytic acid etching but requires nothing but cheap household materials! The hardest part is taping off where you want the marks to be. This works by actually removing metal only where the current flows.
10 gallon kettle, completely bare.
All you need is something acidic enough like vinegar and an electrolyte like salt. A ratio of 1/4 cup vinegar to 1 tsp salt worked for me.
9 volt battery and etching tool
Hook up the POSITIVE lead directly to your kettle.
Attach the NEGATIVE lead directly to a Q-tip, making sure the wires will be touching the vinegar solution.
MAKE SURE YOU HOOK THEM UP IN THE RIGHT ORDER. The other way around would try to ADD metal to the kettle.
This was the only part I didn't have lying around. They were cheap at a local craft store. Adhesive and re-usable, perfect for this project.
Dip the Q-tip in your salted vinegar solution and touch the kettle. If you hear some sizzling or see some bubbling, you know it is working. It only takes a few seconds of contact to permanently dissolve some metal. I suggest testing it on another piece of the same metal that you don't care much about before etching your kettle.
It turned out even better than I expected. No more guessing or relying on a measuring stick.
Again, practice on some other metal first!