Here's what I did to my very old and rusty drift boat trailer, 12 years old and a lot of gravel miles. I took it to a place in my home town and had it sand blasted and then they sprayed on a zinc metalizing that does not rust from the outside. This is the third trailer that I have done it to.
It looks brand new and really not that expensive.
I also put on a dual roller bunk (see picture)because I was having trouble in off loading the boat because of shoulder problems. Now I can roll boat off on a flat surface with just one hand. The best I have ever used. I also had the UHMW bearings made and put metal pipe inside the plastic pipe with a 5/8" stainless shaft. Very easy job but you must have it welded. I have all the extra parts to to this to any drift boat trailer.
The trailer still might rust from the inside out. And the rollers might wear on the bottom of my aluminum drift boat but it is what I had to do.
fp