When I say “harsh” I am not entirely specific. You will be using coca cola, generic or otherwise, and believe me, that stuff is harsh. Hospitals use it to clean out parenteral nutrition tubes when no other industrial strength cleaner will work. It’ll clean a penny, then dissolve it. It’ll dissolve your insides too technically speaking. And your teeth. And hopefully, the scale on the bathtub.
I’d tried comet/ajax to no avail. Then I googled and this idea came up. Here’s what you do.
You will need
Two 2L bottles of coca cola, whatever brand you prefer
a roll of paper towel
Pour the cola into the bucket, then dip each sheet of paper towel into the cola and stick it to the bathtub. Leave on for half an hour, then squeeze out the paper towels and throw them away. Wash the bathtub.
While this method is the only one that has worked so far, it didn’t remove all the scale. There is an improvement, about 65%, but it’s not perfect. I’ll have to do it again. I hate doing things twice. If something’s going to work it had better work the first time around.