That rust is normal if you ride in the rain. High temperature paint works for a while, but soon burns off and rust starts again. I've had good success with ceramic paint from auto parts store. It's dark grey in color. The procedure is to remove the pipe, sand all the rust to bare metal, spray 2 coats of ceramic paint, and then BAKE at 275C or 400F for 1/2 hour. Spray another 2 coats, and bake again. If your muffler is welded to the pipe, it might not fit in your home oven. In that case, look for a commercial ceramic paint shop that does this for race cars, etc.