This is a problem I encounter from time to time. I foget to rinse out the paintbrush after varnishing. The brush isn't completely covered in varnish because I at least remembered to use up all the varnish that was in the brush, but I fogtot to wash it.
But with nothing to lose, I tried the bandsaw to "cut" the brush fibers loose from each other again. I just pushed the brush into the blade and moved it side-to-side while the bandsaw was running.
This works surprisingly well! I have used this trick a few times since then.
Bristles all soft again!