In terms of physically marking metal, metals show fine detail, so proper settings play an important role. Metals mark differently than other substrates like plastic or wood, and may require extra time to perfect settings. Plastic and wood are more about removal; you are removing/burning away one layer to reveal another. If something is not marking properly, the standard reaction is to give it more power.
On metal, marking is a thermal reaction of bonding or changing the coating on the metal. Not enough power and the marked area can be streaky or might not have enough energy to transform the mark to black. Too much power and it can bleed the image or burn through the coating.
-Karly Baldi, Horizons Imaging Group
