use an orbital sander with sticky pads , i never use any filler, use a p80 pad to get rid of all the kerbing, then go over with a less course pad, grey undercoat to cover any imperfections then your top coat.
just seen that the ulez protesters have started putting bat boxes over the ulez cameras , bats are protected by law and cant be touched even if the boxes dont have bats in them and are empty
Genius.