I use a cap full of bleach in the water and I no longer have that rust.If it does appear, I use a tire brush and it cleans right out. That small amount of bleach does not hurt the birds.
And if you still want to get rid of the rust stains, try the salt and lemon juice that took stains out of my bathroom sink where an un-named teenager dropped a metal framed mirror and left nicks in the the porcelain. Just cover with an old rag to prevent the paste from drying out too fast. I later used the same solution of lemon and salt on other rust stains to good effect. The stains on the bricks holding up my flower pot shelves came right out. Also very good other stains like those on old fabric items you just dont want to bleach, like my great grandmother's lace collars and handkerchiefs.
lay a paper towel on the rust, soak with water, let sit, and 10 minutes later, wipe up the rust.
i know this sounds crazy but if you rub a lot of dirt or scrape some sandstone into the cement (instead of trying to get the satin out) you will darken the bowl of the birdbath and it will actually attract more birds. it has a more natural appeal to the birds that way than a pristine new look. you can also add some stones or sand to the bowl.
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