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If you were using oil-based paint then you'll need gasoline or paint thinner. Using gasoline is faster and cheaper, but you can reuse paint thinner by saving the dirty paint thinner in a bucket and letting the paint sink to the bottom. Either way just pour about a quarter inch of the solvent into a coffee can or bucket. Dab and swirl the paintbrush in the solvent until the solvent is saturated with paint. Empty the can and repeat until brush is fairly clean of paint.

If you were using latex-based paint then you'll just need water, unless it's really stubborn you'll need furniture refinisher or lacquer thinner. Firmly rub the paint off the bristles with your fingers under running water. If it's sort of dried up use a scraper- but be careful not to scrape too hard or you'll fray the bristles. If this doesn't do the trick then use furniture refinisher or lacquer thinner with the oil-based instructions.

An old painter's trick if you want to leave a paintbrush for a few hours and not have to clean it, then wrap it up in aluminum foil (or plastic wrap) and leave it in the freezer.