I want to remove a dark brown water stain from my porcelin sink.

Try hydrogen peroxide mixed with baking soda into a paste

Since you've tried all those other things, perhaps you could give Bar Keeper's Friend a try as a last resort. It's commonly available at supermarkets and places like WalMart/KMart near the scouring powders (like Ajax or Comet) and is easily identifiable by the gold can.

I agree with Molly! Bar Keepers is a great product for cleaning porcelain...especially as it ages and pits. This should do the job. If not after using it, fill the sink with warm water and add some bleach...let sit for a bit. An added bonus, throw your dish cloth and sponge in the water!

Bleach, worth a try!

Try Barkeeper's Friend...sold near the Comet and Ajax in most grocery stores.

Bleach!