2 to 3 cups (500 to 750ml) vinegar
No matter how well you wash out a paintbrush, odds are it will have stiffened by the next time you want to use it. Of course, vinegar offers an excellent means of softening and restoring paintbrushes. Place the brushes in a small, heavy-bottomed pot (one that you can dedicate to household situations and not use for cooking) and pour enough vinegar (2 to 3 cups, or 500 to 750ml) over the brushes to soak the bristles. Let sit for 1 to 2 hours. Then place the pot over medium heat and simmer for 10 to 20 minutes. Rinse with cool water, and you’re ready to paint!