Enameled Cast Iron

Two things to try: (1) sprinkle cream of tartar on sink and rub with a damp soft cloth. Wait about an hour and then rinse. Or you can try (2) fill the sink as much as possible with water hot water and about a cup of bleach. Let it sit overnight. Drain and rinse the next morning. I would recommend doing the bleach soak first and then the cream of tartar if needed.

Have you tried Bar Keeper's Friend? or the ultimate is to use Easy Off Fume Free Max oven cleaner.

Be careful not to scrub off the glaze or you get to the porcelain under that and it will ALWAYS stain.

Scratched glaze is probably why it's stained. If you can securely plug the sink try dumping a bunch of Oxyclean into it and filling with hot water to cover the stains. Leave it overnight.

Use Mr. Clean white pads, they're amazing!

a paste made with baking soda and hydrogen peroxide works great. Leave on stained area for a while then rinse. It keeps my 60 year old kitchen sink looking new. Bar keepers friend also works really well.

Your welcome! Cream of Tartar also works on dishes to remove those tiny line discoloration that look like faint, lightly colored cracks that you sometimes see on older porcelain/ceramic dinnerware. I had never heard of using cream of tartar until I found it on Pinterest. Good Luck without your sink!