photochemical machining

It depends upon the metal being etched. FeCl₃ is good for brass. No-one would etch mild steel.

You can use Fluoric acid that will eat away even glass, but be careful. There is the ole Nitric Acid which is used to etch metal for graphic arts and Aqua Regia for gold 2 parts Chloric and 1 part Nitric. If you are etching Aluminum use a milder acid solution as the reaction is vigorous, and if you poor into water (heat steam blow acid in your face) you should not to dilute stop go to school take chemistry 101 because Don't Drink and Mix You'll Hurt Yourself. Phosphoric acid only converts the rust in to an iron phosphate but the change is stable a form and keep surface from rusting anymore but it is balck in color. Muriatic acid (Chloric and Sulfuric mixed to clean pools with) works on some metals and only cleans rust on others but don't leave it too long or promotes rust.

I would say your best bet is Nitric acid, Ferric Chloride is for copper PC boards you want some powerful shiznit OG, Nitric is the shiznit your looking for, ask Billy Maze he'll shout it out.

Always have some water going during a mix to shower yourself if you drop it on your head or something stupid and bad. One false move will spoil your day there can be some gas evolved hydrogen and it is flamable so vent the stuff keep away from flames and sparks.

OK, class is out kiddies remember don't horse around to much Tricks are for kids.