Best of luck with that job! It'll be one of those that go "Oohh...nearly...nearly...damnit! Oooh, nearly again...nearly...blast!!" - a bit like apple bobbing or hooking fairground fish out of a barrel.
If you're going to do it yourself, find a piece of flat wood that will fit down the bore (1cm wide will do) and glue some fine grade carborundum paper to one side of it - I'd use 600 grit - and then use it like you would a file. It'll dress the top of the tube, and should free up any sharp bits hanging off the sides.
The end of the tube is often not flat. And hard materials make a poor backing for sandpaper. I've not tried it but I would be inclined to suggest using contact glue to glue the sand paper - wet&dry probably best, with water as lubricant to make it last - along an index finger and use that.

Naphtha - i.e. lighter fluid - and some rubbing will remove the glue.