Hardened paint can often be removed from a paintbrush naturally using household vinegar.
• Heat white vinegar in a pot on the stove to almost boiling.
• Place the paintbrush in a disposable aluminum baking pan.
• Pour the hot vinegar over the paintbrush.
• Wait 20 or so minutes.
• Then, use a brush comb to remove the softened paint.
• Wash the brush with warm, soapy water.
• Rinse the paintbrush with clean water.
• Allow the brush to dry on a flat surface.