Winsor Newton Brush Cleaner & Restorer
This is the MOST amazing product! Within seconds, horrid, stiff and paint filled brushes loosen up and become flexible. Within a short period of working them, the brushes lose their crust and residue and are restored to a perfectly paint-free condition. You won't believe the effectiveness of this AP-rated (non-toxic) liquid. Thoroughly clean out brushes with soap and water when you are done.
One thing this product can potentially damage is plastic plumbing - so use (and reuse) small amounts of it in larger container, and wipe residue and subsequent soap and water residue onto a Kleenex. This is only necessary if you have plastic plumbing.
For an absolutely over-the-top cleaning, follow your cleaning with another cleaning with Masters' Soap. It will get virtually all tough residual colour from staining pigments like Phthalo out of the brush. A bit of Masters' Soap can then be left in the brush tip and swiped to hold the filament together in perfect form as it dries. Your brush will look and perform as brand new. AMAZING.