You could glass bead them, they should clean up pretty quickly.
Check the mating surface to make sure they are flat.
MBHD
Borla says these are 304 stainless.
Tubes are not magnetic, but flanges are.
I used some fine scotchbrite in the upper photo.
The black is tarnish or some kind of coating, but I think not paint.
The lower pic is after coarse scotchbrite and then fine. That's me in a yellow shirt starting to show up in the reflection. Not sure I need them real shiny.
Maybe some kind of blasting is in order?
Baking soda or walnut shells?
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