It seems like a 5 minute epoxy is the most common choice, but I've seen here where some prefer to use silicone glue. Here's a few links that may help as well - http://theshaveden.com/forums/threads/temporary-knot-set.35658/#post-628509 http://theshaveden.com/forums/threads/brush-re-knot.28901/ http://theshaveden.com/forums/threads/new-knot-in-old-handle.35629/#post-628568 http://theshaveden.com/forums/threads/setting-a-brush-knot-pic-heavy.18202/
I would recommend Devcon 5 minute epoxy Marine Clear at about $5.00 @ Home Depot or Lowes. Or Gorilla Glue Marine Grade Clear Epoxy. At Home Depot $4.95. I would recommend The Golden Nib's 28mm F2 finest knot. Good Luck
Or, you can skip epoxy altogether and use silicone adhesive like I do. Much easier to work with, and you can use a small amount to temporarily test the loft a couple times, if you are happy with it simply pull the knot out and add a little more to permanently set it. I have also changed my mind a couple times and removed the knot and replaced it with one I liked better, something that is practically impossible with epoxy. It also sets up a lot slower so you have plenty of time to fine tune the knot to make sure it's straight and it's a lot easier to clean up if you get some in the hair or on the handle during assembly.
If its sticked with epoxy you can use steam to take it out. Put the brush in a bowl and put the bowl inside a pan with boiling water in it. Close the lid and after ~10 mins. Take it and pull the knot gently. Ps: it does not damage the hairs.
I'd recommend F2 finest as well. Maybe get 2 different grades of badger. F2 for 20mm and Best badger for 18mm or something like that.
O that's just a TGN rating. It's what they named their newer finest knots, F2. Like their original finests would be considered F1. They are just different supplier or something. They have great backbone and are silky soft tips.