This is how I have learned to create etched red brass and copper with the use of pnp transfer film, as the resist and the chemical ferric chloride which eats away the areas not protected by the film.
The metal must be super clean prior to applying the pnp film, the red brass is prepared by sanding with a scrubbie and a cleaner, I use "Barkeepers Friend" or "Penny Brite" ~ the water must "sheet" off of the plate, if it doesn't then it is not yet clean. The metal must be dried well and not handled again until transferring the design.
Press-n-Peel Blue Transfer Film, "PNP" for short, is used in the electronics world, to make printed circuit boards. I find graphic line art and refine it or create my own designs, I then lay it out in my art program to fit a 3" x 6" metal sheet, it is best to use this technique on factory cut material, otherwise there could be dents or divets in hand cut sheet which would leave an air bubble after the ironing process.
Ironing on the transfer image: this process can be "hit or miss", I spritz the cleaned sheet with water and quickly lay the pnp film, dull side down, on top of the metal and press out any bubbles with my hand. Press the iron down onto the film while holding a tab, to keep it from shifting and begin slowly moving the iron around with firm pressure being careful not to shift the film. I use the tip of the iron to "burnish" the film. If the iron is too hot it will melt the pnp - lots of trial and error.
Etching the metal: I apply clear packing tape to the back of the metal and "float" it into the ferric chloride, depending on the age of the acid, the etch time could take up to 3 hours. I run my "Thumbler's Tumbler", which sits next to the acid bath, to "agitate" the acid, little bubbles could form otherwise and detract from the design. You would check the etchings periodicaly for depth, I use a toothpick. When the desired effect is achieved you would remove the tape and submerge the sheet into a bath of water and baking soda, to neutralize. Clean the pnp film off with a scrubbie and acetone. Scrub the material with more barkeepers friend and dry.
I especially love "Tribal" designs, Native American motifs, Petroglyphs, flowers, leaves and abstract images. I've just created a new design that has a tribal feel using bears, wolves, fox, ravens etc. that should be fun to work into a design.
So there you have "Etcing Metal 101" ~ I hope you enjoyed learning about one of the processes that goes into how I create my jewelry.