Tanin stains

A dishwasher!

Mum (or Dad) used to boil up the rhubarb leaves in saucepans to clean them. I only found out years later that they are poisonous!

Tomato sauce cleans coins - apparently

My grandmother had a metal teapot, and she always used to clean it, and discoloured saucepans, but pooling up remnants of rhubarb in them. They came up sparkling, and she would say, with great satisfaction:

"If it does that to a saucepan, just think what it's doing to your insides!"

Think it is oxalic acid that does the job?

Not a lot of use to you, though. Sorry!

Bicarbonate of soda sprinkled on to a damp cloth, doesn't leave taste or aroma.

Diluted bleach and a good rinse.

Steradent!

Sterna dent tablets

Household bleach - easy peasy.

Soak in a solution of bio washing powder - I use Persil, but I'm sure others will work too. And, as others have said, rinse (and rinse again!)

I use diluted bleach. Have done for years.

Obviously rinse thoroughly.

Salt on a damp cloth

Jif (sorry, Cif) on a damp cloth or fill with Steradent, but rinse out the Steradent well - horrid stuff.