I clean the ferrule with alcohol and magic eraser, then I wrap it with a single layer of 1/4" wide blue painter's tape.
Sometimes a little cyano seeps under the end onto the ferrule, but not very deep, maybe 1/32 inch. The tape also serves as a tell-tale when turning down the tip. I put a stop on the carriage to stop the cutter from going onto the ferrule more than about 1/32 inch and when I get a little blue tape picked up by the cutter, I know I'm getting close. At that point I peel the tape off and proceed to finish trimming the tip at .001-002" increments until I"m flush on with the ferrule..
Works for me.
Gary