How to Soften a Paintbrush
If your paintbrushes are hard because you haven't used them in a while, you can use various household items to soften them up. Try applying a pea-sized amount of baby lotion or hand lotion to the bristles. Then, bend the brush back and forth on your hand so the bristles get coated in lotion up to the handle. After about 2 minutes, wipe off any excess lotion with a towel. Alternatively, soften your brushes with white vinegar. All you need to do is boil 2 to 3 cups of white vinegar and put your brush in a jar with the bristles pointing down. Once the vinegar begins to bubble, pour enough into the jar to cover the bristles and leave your brush to soak for 20 minutes. Scrape off any leftover paint with a comb, then rinse your brush under warm water.
Squirt a pea-sized amount of lotion into your hand. You can use any type of baby lotion. However, if you don't have baby lotion, any hand/body lotion that you have in your home will work. The lotion ingredients aren't important, but you should try to use one that dries cleanly. Any greasy residue may damage the bristles. Baby lotion is recommended because of its extra moisturizing properties. Run your paint brush through the lotion. Do this as if you are painting your hand. Bend the brush back and forth, making sure you coat the brush in lotion up to its ferrule (the metal end of the handle). This should take about a minute and a half until the bristles loosen. Wipe off the brush with a towel. Once you are happy with the state of the bristles, remove any excess lotion with a towel. Scrub the towel gently over the whiskers of the brush, starting at the base and moving in circular motions towards the tip. You want to keep medium pressure to avoid plucking or bending any of the bristles. Keep in mind, dried brushes may never get soft. This treatment might help if it is done a few times. Boil white vinegar in a small-medium saucepan. The amount of vinegar you use may vary depending on how many brushes you are attempting to soften. However, you should have enough vinegar to coat your brush/brushes from the tip of their bristles to the ferrule or handle base. Keep in mind that the vinegar will start to evaporate once it begins to boil, so you may need to add more. If you aren't sure how much to use, aim for 2-3 cups. Place your brush/brushes in a heatproof jar. You are going to need to stand your brushes up, bristle side down, so make sure whatever container you choose has enough height. You can try using something like an old mason jar, or even an old, clean paint can. Just be careful as both of these items will be hot to the touch once you pour the vinegar. You can also place your brushes directly into the boiling pot of water, but be extra safe! Pour boiling vinegar into brush container. Once the vinegar has begun to bubble, remove it from the heat and pour it into whatever container you chose. You want to make sure you only pour enough to cover the bristles. If you pour past the brush's ferrule it can loosen the glue that keeps the bristles together. Let your brush/brushes soak for 20-30 minutes. Scrape off any remaining paint. If there is any excess paint leftover, gently remove it with a brush or a comb. You can use either a plastic brush or an old hair brush comb. However, avoid using anything metal as it can bend and ruin the bristles. Start at the base of the handle and slowly comb downwards. If you can't get all of the paint off, simply return your brush/brushes to the vinegar and allow them to soak longer. You can also try soaking the brushes in Mineral Spirits paint thinner to help break down the remaining paint and to try to remove as much as you possibly can. Rinse and apply lotion. After you've soaked and combed through your brushes, rinse them off in warm water. You can softly massage the bristles as you run them under water. Afterward, take a pea-sized amount of baby lotion and work it through the whiskers of the brush in a similar fashion. Condition your brushes. If you still feel like your brushes are hard after rinsing them and applying lotion, cover the whiskers of the brush in hair conditioner. Then, place your brushes in plastic baggies, with all the bristles facing one corner. Once they're all in one bag, seal it tightly. Place the baggie in a bowl of hot water. You don't need to boil this water, just run the hot water tap to bathwater temperature. Then, make sure the water covers the bristles. This will heat up the conditioner, letting it penetrate the bristles better. Leave them to soak for about an hour, replacing cold water with more hot water as it cools down. After they've soaked, rinse your brushes off. Scrape off any excess paint. Before you start soaking your brushes, make sure you try to remove as much paint as you can. You can do this with a brush cleaning tool or with a plastic hair comb. Just make sure that you aren't pulling too forcefully, as this can cause some of the bristles to loosen and fall from the brush. Mix fabric softener and water in a large bucket. Any typical fabric softener will work. Pour a ½ cup of fabric softener for every gallon of water. For example, if you are using a 5-gallon bucket, then you would use two and a half cups of fabric softener. Of course, if you are only softening one or two brushes, you definitely won't need 5-gallons worth. Fabric softener is better than dish soap because it lowers the surface tension between liquids and solids, which means it makes water “wetter.” If you don't have fabric softener, you can also try using laundry detergent. Swish your brush/brushes through the mixture. Taking one brush at a time, swirl them in the fabric softener/water mixture. You want to put them into the solution up to the ferrule and then flick them back and forth quickly, counting to ten. The paint should dislodge from the bristles and fall to the bottom of the bucket. Once the paint is clear, set the brushes aside to dry. 
Squirt a pea-sized amount of lotion into your hand. Run your paint brush through the lotion. Wipe off the brush with a towel. Boil white vinegar in a small-medium saucepan. Place your brush/brushes in a heatproof jar. Pour boiling vinegar into brush container. Scrape off any remaining paint. Rinse and apply lotion. Condition your brushes. Place the baggie in a bowl of hot water. Scrape off any excess paint. Mix fabric softener and water in a large bucket. Swish your brush/brushes through the mixture. 