I had a plisson like that. The knot would get loose in the handle when I didn't use it for awhile. That one would tighten up once I got it wet and the knot would swell up again. The knot fell completely out of my 40 year old Hoffritz brush and I used silicone to glue it back in. I don't know what I would do if I couldn't get the knot out. It would be hard to glue without getting residue/excess on the bristles where they meet the handle.