A can of WD-40.
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How To Remove Pesky Toilet Bowl Stains With WD-40
The multipurpose lubricant WD-40 has over 2,000 uses, including removing toilet bowl stains, and a 3-ounce can of it costs less than $5.
To let WD-40 work without dilution, turn off your toilet’s water supply, and flush it once to remove as much water as you can. Next, shake the can and directly spritz the stains.
Let it sit for a few minutes before scrubbing the stains in a circular motion with a toilet brush. You may need to do this a few times, particularly with older stains.
When the stains are gone, turn the water supply on and flush a few times to rinse away any residue.
In addition to doing away with stains, WD-40 lubricates the inside of your toilet bowl, helping to prevent future stains from sticking to the bowl and seat.