Here's an Opal brand brush whose handle I like, but the knot was a poor performer. It was so thinly packed, I was able to poke my calipers in there and measure the socket, even without removing the old knot first. Then I ordered a new silver-tip knot in the proper diameter.
You can enlarge a socket and install a larger knot if you want, but I thought the original diameter looks nice on this handle.
First, give it a haircut. Get as much of the old bristles and glue plug as you can, without risking damage to the handle.
The glue plug is clearly exposed now.
Carefully drill 4 holes in the plug, in a cloverleaf pattern. Mark the drill bit at the depth where you want the new plug to sit. I went down to the thickness of the new plug, so the glue wouldn't show, but the maximum length of bristle would be exposed. In this case, that would give a taller loft than the original knot, but I thought that would be fine.
Begin removing glue with a small, extremely sharp chisel. Don't hurry. Keep the handle (and your fingers) safe.
Test the fit of the new plug when you get close to the right depth (which you know by the depth of the drill holes). In this picture, I still have more old glue to remove.
When you're very close to the right depth, begin to make the bottom of the hole smooth and level. At first you can use your fingers as a guide; feel for rough or high spots and remove them with the chisel. To finish, place the new knot in the socket. If it rocks back and forth, that means there's a subtle high spot to take down. When the new knot sits firmly in the socket, at the depth you want, you're done with the chisel.
That looks and feels just right.
Time to glue in the new knot. I'm using two-part epoxy optimized for plastics. The Sharpie will color in any chisel marks in the black interior of the socket.
I like to mix epoxy on something non-porous and disposable. Empty food packages can be perfect for this.
Estimate the maximum amount of epoxy you can put into the socket, and not have any come squirting out when you insert the new plug. You don't want any on your new bristles.
Razorburned wrote that you can put a band of masking tape around the bottom of the bristles to protect them from excess epoxy; I found that made it difficult to know for sure that the plug is in the socket perfectly straight up-and-down. I just take everything very slowly and carefully. No problem so far.
Voila! A brand new, vintage brush. Check the epoxy package for the curing time. Don't cheat!
Now, you try it. Let me know how it goes. Post some pictures.
--Glenn
