When I fitted treadwear the sandpaper is for roughage, it's the three scrubbings before hand with degreaser that are the ballache. You need to shift as much oil, grease, ooomscah from the prepared surface as you can before even breaking out the sandpaper. When you think the tyre area is clean, wash it twice more and see the black STILL on your cloth. Also be ready to wipe up excess, my first tyre was messy but you get into it after that.