SonomaProducts.com says...

Test these before trying on real work:::
Another thought is to try washing it with mineral spirits or some other
solvent that won't disolve the shellac. I'm not actually sure that MS
won't dissolve it but I'm sure there is some solvent that would work.

You can use mineral spirits or naphtha to wash shellac. Either works
fine, but I prefer naphtha because it evaporates so fast.