There's a brand of brush cleaner made by Stencil Magic...supposed to be for stencil brushes, but it works VERY well for removing just about any paint or stains from brushes, unless the paint has been in the brush for a very very long time. It also works as a great brush conditioner... I used it to clean a few brushes (natural bristle, sable, and synthetic) that were just stained, and not only did it remove the stain, but it also left the brushes softer, supple, and near perfect...almost like new.
Just my two cents.
Cassandra
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*Are you wishing, or being?*
*Temet nosce.*