Thanks for all the input!
I'm posting some pics - I just scrubbed my sink with an SOS, and these definitely won't come off.
I don't think I've ever taken pics of my sink before :P
Thanks for all your input - any guesses now that you've seen the pics? It's not rust, it's not calcium... It IS a 30 year old sink, so maybe its warranty ran out about the time I got saltwater tanks?
