I'd say just keep it. My boat sees a lot of saltwater use and at some point trailers just become expendable. Most of my friends put their boats and trailers into the saltwater and we do our best to clean them but rust never sleeps! You are faced with trying to get top dollar for your painted trailer and upgrade to galvanized or aluminum. You can get a LOT of use out of your current trailer before it becomes scrap.
I just blasted and painted mine and it will surely rust again but that's part of boating here for us. Use the boat often or sell it, right? Without saltwater use, I'd be lucky to put 20-25 hrs on it a year.
We use one friend's boat the most (4+ times a month, almost year round) and although his trailer is starting to get rusty in places, it is really not bad at all considering how much salt use it sees. And he has been doing this for 5-6 years or more now.
Just my opinion and experience.