Signing Metal Prints

I have an opportunity to display some of my images in a local gallery. My plan is to have the images printed on aluminium. I'm very reluctant, however, to sign the front of the image...just as I'd be reluctant to actually sign a conventional print anywhere other than within the unprinted border area below the image itself.

Does anyone have any practices or suggestions to share? I'm leaning towards signing on the back of the metal print, but perhaps I'm overthinking this.

I've been using Gelly Roll gold pens to sign some limited edition metallic paper prints and they worked great. When I decided to do a series of aluminum prints, some customers asked for them to be signed, so I gave it a shot. I'd say I had mixed success. The ink doesn't take as well to the harder surface, so the writing was much "thinner." With a bad pen, it could look pretty shoddy, so I still don't feel comfortable doing it and give my customers fair warning beforehand.

I imagine there may be a different type of pen that could work better on aluminum, but I've only used the Gelly Roll ones thus far.

Max