It was just a cheap single-use brush I was gonna toss anyway, but I remembered seeing a YouTube video from seejanedrill about using oil-based soap to clean brushes, so I figured I’d give it a shot.
I let it soak overnight in a plastic bag with undiluted pine soap, then cleaned it this morning with a stiff brush. Yesterday it was rock solid - now it’s perfectly soft, like new.
Murphy’s is pretty good but pricey, here. I did a search but didn’t find any USA retailer offering this product. Thank you anyway.