When brushes are left overnight, or longer to soak in water, it can actually cause the glue holding the ferule to the handle to become loose. The two can then become dis-joined from one another, rendering the brush useless. This can also simply happen over time. Most people then throw these brushes away, but why do that when a spot of glue will fix it in no time?
Super glue : the “quick fix”
I am often in a hurry when I run into this problem, and grabbed super glue, which means I can use my brush again in a minute or two. This is a fine “quick fix”, and can last for some time. All you need to do is apply a thin coat of super glue (I prefer the newer gel varieties, but either will do) to the handle, securely replace the ferule, and hold the two together for 60 seconds. I usually give it another minute or two to dry on its own, then I am once again ready to use my brush.
Waterproof glues : takes longer, but lasts longer too
However, chances are the water you use to clean your brushes will loosen the super glue over time as well, and you will be right back where you started. If you have the time, I advise taking it, and using a waterproof glue. Some suggestion here would be waterproof wood glue, Guerrilla glue, or my favorite, though sometimes harder to find, mosaic glue.
Mosaic glue seems to hold well, and is fairly water resistant. Wood glue, though it works quite well, sometimes does not stick to the metal as good as the mosaic glue does. Gorilla glue, though very long lasting, can be a bit messy, due to the fact that it tends to expand while drying.
The procedures for dealing with these particular glues are :
1. Wood glue
The method for wood glue is similar to that for super glue. The only real difference, is that you will want to clap the ferule and handle together while it dries (usually overnight). An alternate method would be to crimp the ferule a bit with a pair of pliers where it’s bottom meets the handle. This must be done gently, or you may break your handle. Either of theses methods may cause glue to ooze from the bottom of the ferule, in which case you will want to clean this off with an old rag or paper toweling before it dries.
2. Guerrilla glue
As mention, Guerrilla glue has a tendency to expand, so you shouldn’t have to crimp the ferule in this case, but you will certainly have to wipe any excess as it escapes the ferule. Again, allow to dry over night.
3. Mosaic glue
I like this glue because it bonds well with both the metal ferule and the wooden handle. By far the best option, in my opinion, the method here is a little different, however. In order to get the two materials to bond well, you must apply the glue to both the metal, and the wood, and allow it to sit and become tacky for a minute or two, but no longer, or it will become too dry. Then you simply secure the two together, wipe any excess from the handle, and allow to dry over night.
These are all good ways to repair your costly paint brushes, and I would suggest trying all three to see which work best for you, if possible. Of course, if you are in a hurry, the super glue will always be your first choice, I am sure. Either way, you just saved a good deal of money fixing your brush instead of buying a new one.
Source : Over 30 years as an artist, crafts-person, and trainer of others in these fields as well.