Oh the memories I have of that. My Rocky a 22 pound long-haired cat had that same problem !! And the knots were horrible. I solved the problem by asking the groomer to shave him totally,well except his whole head and tail. We had to do it about once every 7 months and even though the first time I saw him he looked funny,we all got used to his "new do" in a few days. It was also much better for him because all that nasty hairball vomiting stopped !! And no more smelly butt!! In the past when he had poop stuck on his rear I would have to take him downstairs in the laundry tub and soak the turds off of him.
How he hated that and it was almost a weekly thing !!A job that I did not enjoy either:laughing: He now lives with my son and his wife in a brand new home and my days of dragging Rocky to the groomer are history. I do miss him, but my son and Rocky formed a very strong bond in the 10 years that they both lived here ,so when my son recently married he took his best friend with him