I've also delved into the world of etching metal. A serious technique, care must be taken by way of gloves, mask and good ventilation. I use Ferric chloride as an etchant and am still learning as I go. You can use Sharpie markers as a resist, or Stazon permanent ink. On this brass star, I used Stazon with a rubber stamp. What ever you don't want etched, you need to put on a resist. I also color the entire back of the piece with permanent ink (comes off after with alcohol), or burnish clear, good quality packing tape on the back. Remember, don't ever pour this acid down the sink, it will corrode your copper pipes! Proper disposal by way of collecting in a container and bringing to the local waste facility is a must.