The China Marker pencil (AKA wax based grease pencil) is one of the oldest methods for labeling just about anything! It is inexpensive, easy to use, and reasonably easy to remove, and moderately weatherproof.
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This writes smoothly and clearly. I got it to write on my 10" aluminum garden markers (also available from Metal Garden Markers). It writes clearly, is basically waterproof, but will be easily be changed when needed. If you get the aluminum garden markers, get this pencil, too.