When cleaning soap scum from a porcelain bathtub, I sprayed 409 and then went over it lightly with Scotch-Brite stainless steel scrubbing pads. Not sure how long the scum was there from previous owner, but it came off and didn't harm the finish on bathtub.
By JS from WI
Soap scum is calcium stearate and magnesium stearate. Try using BiowWorx soap scum cleaner. They have laboratory proof they dissolve soap scum. Showers also have what is called sebum which vinegar and most other homemade products can't clean. Sebum is the waxy oil our bodies secrete.