The dreaded gunk.

Humidifiers are wonderful. When you have a cold there is nothing like the misty sound of steam issuing from the humidifier cosily tucked onto the nightstand by your bed. Inhalants gently releasing their comforting vapor...

You know what isn't wonderful?

The gunk that's left on the heater element. Perhaps I should have used bottled, distilled water. Perhaps it wouldn't have mattered at all. Whatever the reason, there is a hard, calcified layer of residue that builds up and renders the humidifier inoperable. And I can't get it off!

So my question is this: When even full strength CLR doesn't dissolve the "gunk", what on earth do I use to get it off?