I have a customer who bought a knife from me about 6 months ago. Knife was D2. 600 grit hand rubbed finish. I etched my logo with an etch-o-matic. Customer emailed me today to say that the knife has rusted around the etch. Is this something that "usually" happens, something I could have prevented, or the result of putting the knife up wet? Either way, how should I fix it?