use lacquer thinner. It is a very fast putty though---don't leave the cap off any longer than nec. .Doing so repeatedly will allow the solvent to evaporate till the mix is , well , poopoo. 

You can use "KleanStrip" brand or similar from your hardware/big-box but the much pricier by volume Tamiya or Testors will perform better . The ratio of solvents is different for dif brands and brand formulas . 

You also may have an old tube.  I went to use some Squadron White Putty (very similar) the other day and it was freakin cured rock-solid with the cap snug super tight !  :