For writing on metal pretty permanently, I would suggest using a Deco brand Paint pen. You can find them in art supply stores, or craft stores. The Sharpie brand is ok. Get the Deco brand, it really is better. Once you get either one, you must first shake it up, like spray paint. Then you have to 'activate' the paint by pressing on the nib in an up and down fashion (use a different piece of paper or cardboard for this) to get the paint to start flowing. Once its there, you might need to shake it or repress on the nib once and a while if you are working on a large project.
Remember that it is paint, so it will take a minute or two for it to dry. But once it is dry it is pretty hard to remove.
BTW, they also come in three line sizes, you would want an extra fine nib.
Good luck with your project