Because aluminum does not naturally oxidize, we use an oil based ink to darken the letters, which should last a very long time if you avoid getting it wet.
If the ink does begin to come out, here are some quick tricks for fixing it:
- Use a black sharpie to fill in the letters.
- Let the ink dry.
- Use a dry magic eraser (like the Mr. Clean ones) to gently rub off any excess ink around the letters.