Somehow I forgot to clean the paint brush after the last trim and shutters touchup. It was an oil based primer and it hardened, rendering the brush unusable. Thinking back, to an accident where spilled MEK had dissolved the linoleum floor tile, I tried putting some MEK into an empty coffee can and soaking the brush in it. Worked like a charm. Brush came out cleaner than when I started the paint job. Note to self, this probably only works on oil based paint.
MEK, short for Methyl Ethyl Ketone. I normally keep it in the shop for use as a plastic cement. I buy it in the paint section of Home Despot.