then rubberized or maybe the asphalt style undercoating. Also I am going to look into that wax stuff they put inside box frames , so that nothing will get stuck in there again and it will last forever that way.

Don't. Just don't. Prep the frame. Paint the frame and then take the best care of it you can. Rinse it often and touch up scratches if you are really concerned about it.

Adding a layer that is not chemically bonded to the metal will just invite more rust. Undercoating will trap moisture, and in your case, salt too. In addition, it makes for working on your rig a real PITA. Filling the frame will have problems too. You are better off keeping it clean and allowing it to dry, than giving it a manner in which to just promote rust.

Good Luck,
Michael