Signing metal prints

I am curious as to what the best way to sign a metal print is. I would think that almost anything would smear if anything touches it. Any suggestions to a specialty marker or pen? Thanks in advance.

Try a Kuretake Zig Writer - they are felt tip, archival, acid free, etc and come in a double tip pen - not sure if, once they dry, they will be smear proof.

http://www.cultpens.com/acatalog/Kuretake-Zig.html