-HCl (For being common and cheap, also known as hydrochloric acid, it's commonly used as a heavy duty cleaner), in case you want to work with copper you can also use Ferric Chloride
-Vinyl adhesive (although other adhesives can be used, as long as they don't get eaten by the acid, the more sticky it is better will work)
Next we want to put the printed image on the vinyl and tape it so it doesn't moves, with a pen follow the lines of the image to transfer them to the vinyl.
TIP: A pen will allow you to tell which zones have been already transferred to the adhesive.
If you have a vinyl cutter, laser cutter or similar this project is perfect for you.
TIP: Aztec format (the one I used) is the tiniest one but you might have troubles with some readers.
Place the plate over a wet rag, don't put it on marble, granite, concrete or similar surfaces because the HCl will eat them away.
Put on your gloves and goggles and try to avoid spills, proceed to pour the HCl in a small quantity, enough to cover the surface, and wait about 2-3 minutes, if you see the reaction goes too fast throw some drops of water, the plate might heat a bit so it's important to keep it cool (that's what the wet rag is for).
IMPORTANT: Keep the area well ventilated, use gloves and goggles for your own safety, the HCl goes easily into the air and can make your throat and your eyes itch, if you make contact with the HCl you will feel an itch and then a burning sensation, try to wash the affected area as fast as you can for some minutes.
General rules and common sense must be applied when working with corrosive substances.
The volume of the solvent (in this case water) will be always larger than the volume of the acid to dilute, never try to dissolve large amounts of acid by pouring water into them.
When it is done place a sandpaper sheet over an horizontal surface and sand the plate so you end with a more defined etch.
TIP: Put a bit of liquid soap inside the etch over the paint so the metal filings get stick to the soap and not to the paint, when you are done just wash it with water.
Polishing it's recommended, you can also use very fine sandpaper instead.
Now you know how to etch aluminum you can do all the etchings you want, from qr codes or identification plates to keychains and decorative objects.