Soften Bristles with Dish Detergent
It seems like even the most careful cleaning of a paintbrush in solvent still results in a stiff, hard-to-use brush after it dries. To keep your brushes as soft and springy as they were when new, try the following:
First, clean your brush in mineral spirits or the appropriate solvent. Then, wash the brush thoroughly with warm tap water and dishwashing detergent. The water and detergent remove minute traces of finish and solvent that otherwise will dry on the bristles and cause them to stiffen.
—Jeff Isom, Waterloo, Neb.