Yep, just keep scrubbing it like dmfa said. I had some spots on mine when they were new, and after a few brew/cleaning routines with a scrubby they haven't come back. If you need them gone now you could get some acid #5 (I think) from five-star and soak with that. I'm pretty sure that's the acid cleaner that re-passivates.
The power to tax, once conceded, has no limits; it contains until it destroys. I was not joking when I told them to dig into their own pouches. It may not be possible to do away with government — sometimes I think that government is an inescapable disease of human beings. But it may be possible to keep it small and starved and inoffensive — and can you think of a better way than by requiring the governors themselves to pay the costs of their antisocial hobby?
R. A. Heinlein