I saw a tip on pinterest that said to soak paintbrushes that have dried acrylic paint on them in Murphy Oil Soap for 24 hours. I have quite a few brushes around that needed this treatment- you know how you get in a hurry and forget to clean your brushes.
Now some needed an extra couple of days and some came off after two, depends on the paint I guess and maybe how long its been dried on there. If it doesn't come off just keep soaking them.
I Love This TiP!