Don.t try to hard to get a good finish.Gal is designed to oxidise.That is the white powdery coating it gets after a few months from new.Anything put over the top of that,will eventually peel off,making it look worst.Every spot that a gal trailer has a drill put through it,will start to rust,because there is bare metal exposed.
There is hot oxidised caustic dips that gal parts go through before powdercoating,but it's not a longterm thing for Galv. trailers used in saltwater.
A good thing I have seen is sheathing the trailer in folded section of stainless or aluminium and using Sikaflex to put them onto the trailer.No drilling and it looks great..
(I have a powdercoating plant
Hope this helps