I know the skin takes a polish pretty well and it stays that way for a while, with castings this does not seem to be the case. I put a new lock in my door grab handle, the one to the right of the door. While I had it off I polished it. I put it on and it looked great but the first time it rained it did not look much better than before the polish. It was starting to corrode.
So the question is what do you do about it.
1. Don't polish and leave it like it is.
2. Polish and it looks like option 1 in a few days.
3. Alodine it and see if that helps
4. Alodine and then follow with clear coat.
I talked to a paint store and they said coat with alodine leave it there for a time then rinse with water. Then coat with $50 a quart clear coat with $32 a can hardener. This would be ok if you had every part on the trailer and you could coat them all at once. I thought Alodine would be a first step and see how long that lasts. If that does not look good then coat with car clear coat after Alodine. The other option is to paint them a uniform gray and be done with it.