There are a couple things you can do for your brush to help resurrect it. First, clip a clothespin onto the brush handle to suspend it over a small narrow glass container. Using the clothespin will keep the bristles from touching the bottom of the container, keeping them straight. Add enough monomer (acrylic liquid) to the glass container to cover the bristles but try not to let it cover any part of the ferrule or handle. It can take a couple days for the product to soften but just be sure there is always enough monomer to keep the hairs of your brush covered.
If that is not working, mix your monomer with acetone (about 50/50). Take the same steps as described above but be mindful that the addition of acetone makes the evaporation rate higher so you will need to be sure there is enough monomer and acetone in your glass container for it to work.
If you allow it to completely evaporate, the product will harden again and you will be back to square one. Once your brush is restored to working condition always be sure to clean it thoroughly with monomer only. Using a good, lint-free wipe during your service will help pull any extra product from your brush when you wipe before dipping it back into the monomer to pick up a fresh bead.
— Tammi Merritt is an educator for Entity