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.
You can do this with old and tired wood decking as well. You clean it off, then you just work some oil based soap into it. Brings it back to life.