Once it’s empty, throw a couple of lbs of coarse magnesium sulfate in there with a little bit of warm Acetone (or if your crud is really baked on bad - some boiling hot limonene instead of acetone). Once you have this mixture in the flask, re-attach to roto and let spin on full RPM without vac. The course epsom salt rocks with act like a pumice to scrape crud off the walls. If your having trouble even with the pumice, u need to Spike the pH to either highly acidic (HCl) or highly alkaline (NaOH)