How do you harden your paintbrush?

To harden a paintbrush, clean it thoroughly with soap and water or the appropriate solvent to remove paint residue. After cleaning, reshape the bristles and dip the brush in a mixture of water and a bit of vinegar or fabric softener, which helps to stiffen the bristles. Allow the brush to dry completely in a horizontal position to maintain its shape. Once dry, the brush will be hardened and ready for use.