My primer brush was nasty and ready to be chucked. The poor thing had been used so many times and then stuck in a bag and put in the freezer. It wouldn't come clean the regular way so I took a tip from Pinterest. I soaked it in Murphy Oil Soap overnight and tadah!, not only is my brush clean, it smells great too.
Now we can get back to priming and painting. Brushes are just too expensive to throw away.