Hello, Jand. Hope am not too late with my answer. Try heating it up it in a vinegar/water mix [almost "boiling" it]. You will need to do it several times [boil, rinse, wipe, boil, rinse, wipe, boil,... with some knocking and shaking in the process...]. I also leave my brushes in a glass with water and a bit of vinegar for a night, after [acrylic] painting; just make sure it's not too much water, just enough to cover the bristles, but not the metal part.