Ok, so i've been clearcoating a headstock i've recently painted with polyurethane. My problem is that when I wash and clean the brush after i'm done with it, it hardens when I leave it to dry. I did just what the dude at the paint store said and washed it in soapy water so I'm not sure what I'm doing wrong here? Am I just not cleaning it good enough? I don't see how that can be the case as I've given it a very thorough cleaning. Should I just leave it in the water?? Please help!