It is common in old acrylic paints for the pigment to separate from the liquid compound that holds it together. In most cases, if it the paint is not hard, it can be stirred back together. If the paint is crusty but still has an ample amount of liquid parts that can be salvaged, use a palette knife to separate the crust from the good paint and place the color you are saving in an airtight tube or jar until ready for use.
Hard lumps or paint that is so crusty that it will not come out of the tube is a sign that is ready to be thrown out. There is no way to liquify acrylic paint once it is hard or crusty. Since acrylic is similar to plastic, spraying water on dried paint will do nothing to bring it back to its original state. Oftentimes, old acrylic paint can become musty and smelly, which is a sign of mold. Do not use paint that has a moldy smell, or the colors will not be guaranteed on your finished painting.
One of the first signs of an old tube of paint is that the lid is virtually impossible to open, even with pliers. Soak the top part of the tube of paint, meaning the lid, in very hot water for up to three minutes. Wipe the lid completely dry and try opening it with a pair of pliers or a rough towel. Do not use your teeth. Oftentimes, the lid may indicate the paint tube is very old, but once open, you may find a fresh amount of paint waiting to be used.