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I don't know if they have a special name.
They are just called 'polijst middel voor staal om op een polijst wiel te gebruiken' and I buy them at the 'legerstock' where my parents live.
The shop...

Another good thing (which I use) are those little green bricks of chromium oxide which you can buy at some hardware stores.
They are used for polishing steel, and often used on buffer wheels or...