Try soaking and working the brush back and forth in some d-sperse to see if it removes it. Whatever you do, DO NOT use acetone as it will wreck the brush for good. If d-sperse doesn't work then you'll have to get a new brush. I had some left in mine but d-sperse got rid of it. Took a few goes of soaking, wiping the brush out on lint free pad, soaking again etc but it worked
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