When we bought this house the bathroom sink had terrible hard water buildup. I tried a bunch of different things, including rubbing the enamel off in certain spots!
I came across a pin that said to soak paper towels in vinegar so I figured why not.
This is a during picture, you can see the brown crud to the left of the faucet. (I always forget to take before pictures)
That first time took a few soaks but it worked! I do it about once a month now. I usually just soak it for about 20 minutes while I clean the rest of the bathroom.
Now I need to find out how to get crazy glue out of the sink!