I also had a 40' United goose trailer that I bought used. I buffed the sides numerous times to keep it clean and white. But the screw heads were rusty. So I stopped by Littons place in Indy one day when I was up there.
Showed him what kind of screw I needed and how big my trailer was and he estimated I that a half to 1/3 full 50lb box would be enough. I forget what I paid him but it was not much.

It took a couple of us 2-3 nights to replace every screw on the outside of the trailer. WOW! What a difference!! I mean big time, shinny screws down the side of a clean white trailer made it pop going down the road. No more rust stains down the sides. Felt like a new trailer and sold it a year or so later for more than I paid for it.

Litton gave me enough screws to do 3 40' trailers as it turned out, so I have using the extras for 20 years now. Have used them on RVs too, same kind of screw is used.