I'm not a master woodworker, but by now I've made a few furniture and decor pieces, and a little actual art, that I'm not ashamed of. However:

I give the latest doodad a coat of polyurethane. I pour mineral spirits in a can. I slosh, and squish, the brush in the can quite a bit. I wipe it on a rag. I empty the can, refill it, and repeat the same. The next time I pick up the brush, it's kind of a thick scraper. What am I missing? do I have to wash it for at least half an hour, or use six batches of fresh cleaner, or what?