Ething a metal can be used to add texture, transfer imagery, or deepen existing imagery.
Rule 1: Etchants will etch you. Take precautions. They will also etch clothing, tools, and surfaces.. Strong etchants like Nitric Acid will cause permanent scarring and blindness. Even weaker ones can stain skin and clothes. Wear appropriate protection for the strength of the etchant, my minimum protection is safety glasses, nitrilite gloves and a shop apron. I etch with ferric chloride... if you use plan to use a stronger then upgrade to chemical gloves, arespirator and a rubber apron If you are using an acid keep baking soda around to neutralize spills and splashes .
For an effective alternate etchant not as dangerous as Nitric... but supposedly more effective then Ferric see this indestructable:
Rule 2: Store and etch in appropriate containers. I etch in a pyrex dish, used etchent goes into an HDPE bottle for later disposal. Since I don't know, how ABS or other plastics react to Ferric I avoided them.
Rule 3: Cleanliness is next to godliness. This mean you metal MUST be clean. Any surface contamination can act as a resist, which can cause result other then anticipated. This ALSO means clean up/neutralize your spilled etchant quickly to avoid staining/etching something other then the target piece.
Rule 4: Anywhere there is not a resist will be etched. Resists can be anything that prevents the etchant from touching the surface of the metal. Examples of etchant: PnP blue transfer paper, fingernail polish, tape (waterproof), and even sharpie mrkers.
Rule 5: Shake it like you aren't supposed to shake a polaroid picture! Okay...you don't actually shake it. But if you place the piece to be etched with the side to be etched face down and occasionally agitate the container there are two benefits. First the etched material will occasionally form a precipitate which can stick to the piece and slow down etching. Agitating allows that to follow away from your piece. The second advantage is it keeps fresh concentration of the etchant in contact with your piece.
Rule 6: Rinse! Rinse! Rinse! When the proper etching amount is reached you need to wash the piece or otherwise neutralize the etchant.
Here is a scenario of the steps I go through while etching.
Tweezers (copper), Etchant, Electrical tape, Pyrex dish, needle probe (made out of heavy gauge copper wire), safety equipment, and the piece to be etched.
Step 1: Put on saftey equipment.
Step 2: Clean the metal.
Step 3: Transfer the image (either drawn on with a sharpie or PnP image whatever...)
Step 4: Cover every part Not currently being etched with a resist.
*whoops Time for Math test... To be continued...*