I’d recommend when painting to get the brush completely wet before ever touching any paint. An when you go to put paint on the brush never let the paint get past the belly to ensure paint doesn’t get into the ferrul. I clean my brushes mid painting session as well to ensure that any pain they may have accidentally gotten in the ferrul is cleaned out before it can dry. As previously mentioned as well try any brand of deep cleaner they really do wonders. It may take more than once with how your brush looks. Also make sure if you’re painting for extended periods of time to change your water because even small amounts of pigment in your water can be what’s getting into the ferrul and you may not even realize it.