If your acrylic paint has become sludgy, lumpy or thick, You can add a small amount of water to the paint to thin it out, or add a medium such as acrylic gel or flow improver to improve its flow. Mix the paint thoroughly using a palette knife or paintbrush, and strain it through a fine mesh strainer or cheesecloth to remove any lumps or impurities. If the paint is very thick, you can try using a paint thinner such as turpentine or mineral spirits to thin it out, but be aware that this may affect the color and consistency of the paint. It's always best to use high-quality paint and store it properly to prevent it from becoming sludgy in the first place.