Spreading glue is a necessary task in woodworking, no matter your choice of glue. If you use PVA glue then a disposable brush is probably the best choice. You may not even use a brush, I have spread a lot of PVA glue with a sliver of wood scrap. Hide glue is different though.
When hide glue is at its proper working temperature it is much more viscous than PVA. This viscosity necessitates the use of a brush for applying the glue. The brush doesn’t need to anything fancy. The bristles need to be stiff enough to push the glue into the joints and the brush needs to hold enough glue so that you are not constantly going back to the glue pot. I like to think of it as a glue mop more than a glue brush.
You can purchase a purpose-built hide glue brush, here and here. They are not too expensive, but it seemed to me that making one or two shouldn’t be too difficult. So that is what I set out to accomplish.
The first thing I needed was a source of bristles. Traditionally horse hair or hog bristles were used. I don’t have a source for either of those, so I headed to the Home Center. There I found a $2, natural bristle paint brush. These are the el-cheapo, throw away brushes.
Once home, I dismantled the brush to get at the bristles. First I removed the metal ferrule.
This revealed that the bristles were glued around a small rectangular wood core. Using a knife, I separated the clumps of bristles from the wood core.
Hide glue brushes are traditionally round. So now I needed a handle. I have lots of small bits of pine and oak in the scrap pile. I ruled out the oak for fear of the tannins causing a reaction with the glue and discoloration. Ideally I would have had some bits of birch or maple, but pine should work fine.
I used a spokeshave and knife to round and taper my handle blank.
I searched far and wide on how these brushes are traditionally made and came up utterly empty. So my method may or may not be correct, but it works just fine. I left the scavenged bristle clumps in their original glued together state. To fit them to my new handle a carved a recess in the handle. The shape of this recess allows the glue clump to sit flush with the surface of the handle. The recess is also shaped in such a way that the bristles will be positively secured to the handle. The remaining core, that the bristles surround, will create a hollow area within the bristles that will hold a fair amount of glue. This it what the recessed area should look like.
I then simply applied enough of the bristle clumps to fully encircle the handle. I then secured them in place with a constrictor knot tied just below the original clumps of glue. This draws the bristles down below the bulbous tip of the recess as well as flaring the bristles outward creating the hollow, glue holding center.
Using a sharp knife, I trimmed any bits of glue clump the were proud of the handle surface. The smoother the transition between handle and bristles, the easier and neater the next step will be.
To complete the joining of the bristles to the handle, I applied a common whipping. This is simply a bit of string tightly wrapped around the joint. I used unwaxed cotton sail twine, but any strong cotton string will work just fine. Most grocery stores in the US have cotton butcher’s twine which should work just fine. I wouldn’t recommend any coated or nylon string. Coatings could contaminate the glue and nylon tends to react unpredictably to heat.
I also added a hole and loop of string at the far end of handle so that I can hang the brush from a peg/nail.
I trimmed the bristles of my new brush to a shape that I think will work best for spreading glue. This will be trail and error as to what shape of brush actually works best for spreading glue into joints though.
The last step was to coat the wrapped joint with a bit of hot hide glue.
There should be no need to clean the brush after every use if you are using hot hide glue. Simply allow the glue to dry. Stick the brush back into the pot on the next round of glue up and the hot hide glue will reconstitute the glue on the brush. If liquid hide glue is your preference, then cleaning of the brush after every use will be the way to go.
Here is a reference drawing that outlines the steps.
Obviously you can make these brushes any size you desire. You could also refine your handles to the nth degree, if that is your inclination.
At any rate, I hope that you found this useful.