When should I "sand" using sand paper before I paint?

There is a black strip of metal between my driver’s side window and the car door. The black strip of metal had been losing its paint such that you can see the silver metal underneath. Today I sprayed flat black paint onto the black strip of metal and the paint did not adhere to the black metal strip (you could still see the silver metal).

Given this, I had read that sometimes “sanding” with sand paper and then spraying the metal strip would help the black paint adhere to the metal strip. Is this true? If not, what should I do so that the black paint would adhere to the metal bar?