The magnesium plate is used for printing and it has a photoresist already applied , all you do is print a positive or negative on transparency and expose the plate to uv with the print atop the photosensitive resist. You then wash out the plate , which removes part of the resist where you want the etch to happen. The plate is dipped in acid or some other mordant and the unprotected areas are eaten away. The reason magnesium is used are to do with etch times and the depth of etch as well as its heat transferrence characteristics. The dies need to be etched deep quick , the longer the stay in the etch , the more chance of the resist being damaged (the resist is what protects the unetched bits) and the more chance that undercutting will occur (the etch pattern is not straight down , it tends to etch sideways into vertical thin sections.)
