Here's the brush I started with. It's hard as a rock.

Cheapskate’s Guide to Reviving a Dead Paintbrush

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Good paintbrushes ain’t cheap, which is why you should wash them right when you’re finished using them.  But every once in a while laziness makes fools of us all, and we find ourselves with a good brush gone bad. This is the process that I use to bring a brush back from the dead.

1. Soak a Stiff Brush in Boiling Vinegar

If your brush is encased in dried paint or stain, never fear! You can use good old-fashioned white vinegar to loosen it up. You’ll probably be doing this in your kitchen, so make sure you have a fan in a window of otherwise ventilate your space as well as you can.

It’s probably best if you find an old pot or pan that you don’t plan on cooking in again since you’ll be removing some pretty nasty chemicals form your brushes.  Fill the pain with an inch or so of white vinegar and bring it to a boil.  Let the brush soak until it begins to loosen up, then begin to swish it until the dried material loosens up.  Eventually the bulk of gunk on your brush will flake off. Once the bristles of your brush bend freely in the pan, you’re done with this step.

2. Scrub with Wire Brush or Painter’s Comb

Now that your bristles are soft enough, begin scrubbing the brushes with a wire brush or painter’s comb parallel to the bristles. When there’s no chunky residue left, you’re finished with this step.

3. Soak in Brush Cleaner

Make sure you use some common sense and wear goggles and gloves when handling these cheminals. Put enough brush cleaner in a mason jar or metal bucket to submerge the bristles but not the metal band. Let it soak for a few minutes, then gently begin swishing the bristles so the liquid can soak in between them.  Eventually the bristles will regain the flexibility they had when the brush was new.

4. Comb the Brush

In a sink under warm water, use a painter’s comb to remove any remaining material from the bristles.  Make sure to get deep into the center of the brush.  The bristles will eventually regain their flexibility and original color.  Make sure to take your time on this step so that you remaining crud dries in your clean brush and ruins your next finish.

5. Spin Out the Water

Companies make tools to do this, but I find that if I keep on the gloves from the last steps and rub my palms together with the handle of the brush between them, I can spin it fast enough to shake the bulk of the water out.

6. Wrap Brush and Let it Dry

Finally, wrap the brush with a sheet of printer paper or newspaper and tape it shut. As the bristles dry they’ll dry straight.