My cat’s got “runways” in his fur. What can I do about this?

Dear Most Esteemed and Knowledgeable Kitties:
I have two cats, Sergei and Buddy, both male, both fixed. Sergei is around seven years old, Buddy is probably around two or three. A little history: I’ve had Sergei since he was around 9 months old. He was rescued from a cat hoarder’s home, lived in a shelter for [...]