AA, at some point these things (and _not_ just horse trailers; rusty objects in general) have a negative value, in that they will cost more than they're worth to fix, and then they either rust quietly away while decorating your yard, or you have to pay someone to haul them away for you. I have a Subaru that has achieved this status, and where I would usually load it up for its final trip to The Crusher, I don't have a title for it, and now-a-days, you have to have a title before the scrap yard will take it. I suppose I will have to take a Sawzall to it, and cut it into bite-sized bits to make it go away. I did recently purchase its twin (but w/o the rust), so I may yet be able to pull some bits and pieces of salvage from it, but overall . . . Sigh.
And Rust Never Sleeps. Even if you do make the Herculean effort to eradicate the rust in one area, absolute guarantee that it is plotting a return performance elsewhere.
Sorry about your trailer prospect, but IMO it just isn't worth the trouble to try and put in a fix once it gets started.
Steve Jernigan KG0MB
University of Colorado