To dissolve rubber cement, you need a solvent. Solvents are chemicals that mix with other chemicals, and in rubber cement’s case, ethyl acetate is an ingredient used in nail polish remover.  Although nail polish remover can damage your hands, it is also very effective at dissolving rubber cement. However, there are safer alternative solvents like acetone or Xylene. If a few drops of nail polish remover remove all of the rubber cement residues, then more acetone will remove it even faster.  This same principle applies when thinning oil paints. You have already thinned your paint to what you think might be acceptable consistency. But if it is still too thick, add a bit thinner (acetone) until it looks perfect.