Soak gunked-up nylon brushes in hot vinegar for up to 30 minutes to remove paint and soften the bristles. Afterward, wash them in hot, soapy water, brushing off paint as needed, then rinse and let dry—good as new.
Seriously?! -- Vinegar works as a wood stain when you add a metal object in the mix and let it sit in a glass container for a week. A handful of pennies will produce a beautiful pale Caribbean blue stain. A wad of steel wool will give you a rich reddish hue. A combination of tea and a metal object in vinegar will produce a black stain. Log this away for later.
To paint over ANY surface, without sanding, use a product called 'ESP Easy Surface Prep', available at hardware and paint shops. Wipe on, wipe off and paint in 90 minutes. Works well over enamel paint , without having to sand. SAVES HOURS of work. It was recommended to me by a professional painter.