You want Methyl-Ethyl-Ketone (M.E.K. ) sold in gallons dirt cheap in the paint supply of most big-box home improvement centers and better hardware stores. It also works great as a glue.
Insanely volatile so decant it into lots of smaller bottles and seal well. Do all your pouring and mixing outdoors this stuff will melt your brain and blow up if given a chance. That said, it is exactly what you are looking for, just handle carefully.
I used to grind up old sprue with an old-fashioned pencil sharpener, the hard styrene stuff (Hasegawa, Tamiya, Monogram) works best, I'm not sure about the waxy stuff Renedra and GW uses