I noticed that the mirror in our shower was really filthy with soap scum, especially at the bottom. And so I got out the paint thinner to clean it.
Let me take you on a very short chemical journey.
What is soap scum? Well, first let's start with this: what is soap? Soap is the sodium salt of a long chain organic acid. Huh? OK, I guess some definitions are in order. "Organic" doesn't mean pure, or natural. Instead it means that it is a carbon-based compound. Long-chain means, well, it is long, skinny molecule, made up of a chain of carbon atoms strung together. The acid part means that on one end of this long stringy molecule, there is an organic acid group: C=O-OH, that has been reacted with a strong base, sodium hydroxide in this case. It is useful for cleaning up greasy things, because the long organic end of the molecule bonds well with the grease, and the ionic acid end dissolves nicely in the water - thus making the grease sort of water-soluble.
OK, so now we know what soap is... what is the scum? Well, the scum comes from "hard" water - that is, water that contains calcium and or magnesium ions. See, these ions replace the sodium in the soap molecules, turning the soap into the calcium and magnesium salts of those long chain organic acids. So what's the rub? It's this: the sodium analog is water soluble. The calcium and magnesium versions are not. So a precipitate forms: SCUM. It feels kind of greasy (that's the organic end of the molecule), and doesn't wash off with water (duh - it's not water soluble).
People spend lots of money on weird chemicals and abrasives in trying to remove this stuff. The grocery store remedies all try to work with the tiny ionic end of the molecule; that is not a very effective approach. The paint thinner works on the 99% part of the molecule: the organic end. A quick wipe-down with the paint thinner removes the scum amazingly easily.
The scum just wipes off. Really.
I know I've mentioned this before. But really, it bears repetition.