Could you describe the process: did you need to justify the time spent on refactoring in any way? ("why known bugs, requested new features should wait until the refactoring is done"--I'm playing devils advocate here. I'm interested, how you justified it before the management if you had one)

I never asked permission to do it. I consider refactoring to be part of my job. Small refactorings I do right away when implementing a new feature. Large ones I split into many small steps and work on for months in-between working on new features. Never underestimate the power of making a small improvement every day. I make sure that the long term refactorings never breaks the system or introduce new bugs. I have solid tests in place making sure changes doesn’t change behaviour.