I think I'd ask boatmate to send out new pieces and get a welder to remove/replace then prime/paint with rattle can. sanding off and redoing my gut is you'll be doing the same thing in a few years and who knows if the structural integrity of the piece is still there to hold up a 5000lb boat.
with a trailer less than 5 years old I would think boatmate would send out the metal hangers for someone to fix. at 8 or 9 years might be a harder sell.