Assuming you take excellent care of your brushes – wiping them carefully, washing them in warm soapy water after each use, it’s still common for paint residue to build up around the ferrule* over time. I find many beginning acrylic painters rinse their brushes in water without using soap. The paint comes out but the clear acrylic polymer stays in the brush so they look clean but the bristles dry stiff and the brush is unusable.
I used to think when oil or acrylic paint dried in a brush and it became hardened that it was a lost cause. I tried lots of things – soaking them overnight in soap or solvent, depending on whether the paint was oil or acrylc. I experimented with all kinds of specialized brush cleaning products such as Kiss-Off, the Masters, EZ-Air cleaner with limited success. The bristles were still stiff and some of the paint would come out but not all of it. I also tried to get dried acrylic out with the soaps many artists recommended such as Fels-Naptha, Murphy’s Oil Soap without success. Once a brush became hardened, I had to throw it out.
It wasn’t until Winsor and Newton came out with their Brush Cleaner and Restorer that I found the perfect solution for reviving old brushes. What I really love about it is that it is environmentally friendly as it is both non-toxic and biodegradable. (But remember the paint and pigments you remove from the brush might not be, so once it is used, please dispose of it in your local Household Hazardous Waste facility.) I reuse the brush cleaner over and over by allowing it to stand in a clear glass jar. The pigment particles settle to the bottom of the jar and the liquid at the top can be poured off and reused for the next cleaning.
* Watch the video to find out what a ferrule is!