Friction fit saya Japanese Knives.
As far as I know, unless it is a very special knife, the sayas are contracted out in bulk adn thus not custom made for that very knife. Being handmade knives they vary just a little bit and it becomes friction fit by chance how much and dont know where the friction action is? and it is left to chance adn if it is too much.. it will cause deep scratches and not too good for pretty knives.
when you have to exert force to pull out a knife from a saya.. can be a dangerous.
I wld round off the spine and choil and spine first before I do anything to the inside of the saya unless there is no such intentions at a later date.
Just my thoughts.
rgds d