I have a confession: I'm a bad brush owner. I own 30 or so brushes and about 20 of them are in disrepair. Paint dried to the ferule, paint dried in the bristles, glue dried on them, matted medium used with a good watercolor brush*. I go through phases where I'm working watercolor or gouache exclusively and washing the brush thoroughly after use is not as necessary as it is for acrylic. So I got into the bad habit of allowing my brushes to dry after only rinsing them.
My painting professor would kill me. He allotted time at the end of each class for all of us to clean our brushes. He spent about half a class period teaching us proper brush care. This was a man who took brush cleaning seriously. His motto was that a expensive brush should last you a life time, if you cared for it properly. If he saw the state of my grumbacher brushes, he'd surely give me an F.
Yesterday, while working on a art journal spread I got frustrated, downright pissed off at myself, as I looked through my brushes, saw that I had several hundreds of dollars worth of ruined brushes. None of which I can afford to replace right now. The only brushes I religiously clean as of late are my liner brushes. I can't paint an entire painting in liner brushes… (Or can I?) So I set about to clean them.
I know what your thinking, its acrylic, those brushes are done. Well as a practiced bad brush owner I can tell you that's not true. Many things will soften acrylic paint. My favorite mix is 25% dish soap, 25% green works cleaner (or some other cleaner without ammonia or clorine) and 50% water mixed thoroughly in a salsa jar. I put about an inch of solution i there. Hte percentages don't need to be accurate, just eyeball it.
You then need to secure your brushes so that they brush part is not resting on the bottom of the container and that the ferrule isn't submerged either. It's a delicate balance. I used blu-tac to hold each brush to the jar at just the right height. You can use whatever works best for you.
For brushes that aren't severely caked with paint/ crud you can try rinsing them out in a few hours. Once the dried on paint is soft you can then wash as normal.
Most of my brushes were ready to go in about 2 hours. I had a handful that I had to let sit over night and I've got 3 I'm letting sit for a week.
I've learned my lesson, from now on at teh end of every painting session, no matter how late I'm washing my damn brushes.
*believe me when I saw that it puts he most perfect amount of matte medium down for an image transfer ever!