Acetone = fingernail polish (enamel) remover. So there you go.
I know I'm splitting hairs here but AFAIK, nail polish is a lacquer, not enamel. I should know - I use it (for painting model cars).
Dry lacquer can be re-dissolved using a solvent (like acetone). Theoretically, if you got the proportion of of solvent right, you could the re-use it. If lacquer thickens up in a bottle (due to solvent evaporation) adding mode solvent makes it usable again.
Enamels are different. They dry in stages. First the solvent evaporates and they can be handled, but the coating is still soft. Then they continue to harden by the process of oxidation. Once the enamel is fully hardened, it cannot be redissolved. Solvents will swell and wrinkle the dry enamel but it will never turn back into homogeneous liquid paint, no matter how long it sits in the solvent. If enamel thickens in the bottle (due to solvent evaporation and air (oxygen) intrusion, adding more solvent will not make it into usable paint. It is unusable.