Scale Removal
I used a non-scratching tub scrubber and "The Works" toilet bowl cleaner. The lime scale was so thick, that it required some heavy artillery! Squirt on, let sit a few minutes, scrub off what will come off, rinse. Repeat.
It takes a very long time, lots of scrubbing and elbow-grease, but well worth it.
The cleaner fumes very badly, and on the REALLY icky parts, it actually smokes! Make sure if you use this stuff to do it outside when there's a breeze! SERIOUSLY! OUTSIDE! Do NOT inhale the fumes as they are hydrochloric acid fumes! I can't iterate enough.....ventilation is IMPORTANT!
I will state one thing: If the metal is not in very good shape in the tub, I wouldn't suggest using the toilet cleaner on it. It's VERY harsh and can damage thin or rusty metal. I was able to do it in this tub since it's porcelain lined, and was actually in very good shape! The thick lime scale actually seemed to preserve the finish somewhat.