Has anyone tried the technique shown here? http://makezine.com/projects/make-40/etch-a-kettle/
Basically you combine vinegar and salt, attach the item to be etched to the + of a 9V battery, attach a q-tip to the - pole (exposed wire wrapped tight around the cotton part), dab the cotton in your salt/vinegar mix and then paint on.
Seems like a bit neater than masking and immersing in diluted FeCl.