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If your paint has hardened completely, it won't be possible for you to revive it. However, paint that has thickened and has become firm but is still somewhat tacky or malleable can often be restored. You can gauge your paint with a poke from your finger or a tap from your brush or palette knife. For paint that is especially solid, press onto it firmly with your finger, the handle end of your brush, or your palette knife. If you notice an indentation form, this is a good sign that you may be able to revitalize your paint. If you notice your acrylic paint has begun to solidify, you may still be able to bring it back to a workable state. Add water or a thinning agent and firmly mix it into your paint with a palette knife. Make sure you have a good grip on your palette before attempting this; this may take some extra oomph, and accidentally knocking your palette on the ground can create a big mess. You may find it easiest to set your palette down on a sturdy surface before attempting this. You should still maintain a good grip, as the smooth surface of your palette will be prone to slipping or sliding as you grind at the paint with your palette knife. If your poke check has revealed that your paint, while significantly hardened, is still malleable, you may not be able to revitalize it by mixing it as you would conventionally. In these cases, you should grind your palette knife to mix water into the hardened paint on your palette. This motion will force the water throughout the thicker, hardened parts of the paint. If after some minutes you notice no difference in the consistency of your paint, it is likely too dry to revitalize.

Summary:
Identify paint you can save. Use a palette knife to revitalize stubborn acrylic paint. Use grinding motions for especially hard paint.