Bob, The undercarriage on my convertible was coated in dealer undercoating and all I have done is clean it with mineral spirits. The screws are nickel plated or whatever the shinny finish is and the brackets are bare metal, no pitting but there is some discoloration probably from more than 50years coated with undercoating. All are positively original on this car.
This is what the same area looked like before and after cleaning. You can follow my ongoing undercarriage cleaning in the restoration section to see how much of the rest of my undercarriage has been nicely preserved.
Wow what a striking positive difference IMO given the before and after pics. I have done it before so I appreciate the effort.