My butt cleaning trick, even though our cat is super vain/thorough on cleaning but sometimes just doesn't do as much diligence on the butt area is just taking a wet papertowel and swiping in that general area. It's super awkward but often the feeling of wetness prompts the cat to go back there and "undo what you just fucked up" better since they generally don't like being wet/moist. It might not work for every cat but it's what I used to do to train my cat when he was a kitten to clean his butt.
Necroa
