Questions about acetone and nail polish?

The solvents in nail polish and nail polish remover (that's the ethyl acetate, butyl acetate, isopropanol and acetone) are the materials that do the work of thinning the polish and dissolving the hardened material are also effective in removing some natural oils present in the nails which keep them supple and prevent them from breaking. For this reason the formulated products contain oily substitute materials to maintain the condition of the nails. If you simply add solvent to thin out old polish you have no guarantee that the proportion of these oils are what the manufacturer intended. Purpose made thinners include oils.