UPDATE!! Believe it or not.
I had to buy some degreaser for another project. After hardly any consideration at all, I grabbed a bottle of Goof Off. Although there's still a fair amount of rubbing involved, I have every confidence that, within a few years, my tub will be clean.
I Have Scum!
Re: I Have Scum!
Tom, just use the Goof-Off on yourself and you wont need the tub.