Buy a small sandblasting gun at Harbor Freight or wherever. yeah it'll take awhile, but it gets the job done. We rented an industrial blaster from a heavy equipment rental place and sandblasted Dads 55 chevy frame. After that we used a grill scrubbing pad with a handle on it and some acid to wash down the frame, then rinsed with plenty of water. After that, apply epoxy (PPG DP90). The stuff is as tough as nails, but its also under a full body, so we werent concerned about fading. In your situation, Id still use an epoxy primer, considering the high salt content your car will be exposed to. I wouldnt really recommend an etching primer, because these are lacquer based, and you'd want to put an epoxy over it as a sealer anyway. Sandblasting or grinding on it is going to be plenty of mechanical adhesion to begin with, and since the frame will be exposed, I would probably use a metal impregnated bondo or Rage gold to slick it up. The metallic filler I apply on bare steel, any Bondo type product, I prefer to apply it over the epoxy primer, since this allows the entire frame to be encapsulated in water"proof" epoxy (Note that you cant apply any filler over the top of an etch primer) Thats just how I roll; theres a million ways to do it, and most will work. Kudos to you on taking on that project, you are doing great and Im sure its going to be top notch