Whatever you do copper and brass will begin to tarnish as soon as you finish polishing them.
A clear coat of laquor (nether I or my computer know how to spell it) will help but it is soft and will scratch and the
mod will tarnish where it scratches and you will have to take all the coating off to polish it again.
For me it is more trouble than it is worth and I have learned to embrace the patina. Except on the
threads where I do clean and then use noalox to help the conductivity.