When my father had this problem on the rear bumper on an old Ford van, he borrowed his brother’s sandblaster, sandblasted it, primed it, and painted it. If you don’t have access to a sandblaster, you might get some sandpaper from the hardware store or an auto parts store, sand it down, prime it, and paint it. Walmart has a product called rust converter, which, after you sand the rust off, makes a nice primer coat. Then you could use a spray painting machine and some Rustoleum or just a can or two of spray paint. If you remove the bumper, or mask the surrounding areas well enough, it might look halfway decent.