I don't know of any emulsion that will protect the metal from an acid. If you are planning on light etching of the surface you can print a resist, most use an aggressive baking enamel then dip in an acid bath. Most acid bath dipping takes place under 30 seconds so the ink will hold up. any longer and you would have to check the tech sheet on the product. This process is also where the phrase handle with kidd gloves comes from. Open pore metal is like a sponge for oils and dirt so no mater how clean your hands are you can not touch it. Kidd gloves are glove designed to protect the metal from the oils in your hands. You will need to seal the etched metal after you are done.

Good Luck