You will need:
1 lemon (more acidic varieties are best)
1 tbsp table salt
elbow grease, but only a modest amount
Instructions: cut the lemon in half. In the bottom of your pan sprinkle half the tablespoon of salt. Rub the lemon all around the dull stains squeezing as you do it to release the juice and using the salt to scrub the surface of the metal. When you have scrubbed enough so that no dry salt remains and the lemon is squeezed out. Let the lemon juice and salt sit for a few minutes.
Next: rinse the pot out and gently scrub at dull stains with a non-scratching scrub sponge. If it’s come completely clean then you’re done!
So if the stains didn’t come completely off the first time: rinse and repeat, as they say.