Some reviews say this does not remove stains or marks left under flower pots. Yes, it does. I have over 30 flower pots of all sizes, even small trees. I have successfully used this on my balcony for 5 years and my balcony marbles are shining clean, also under the pots. But you have to use it as any other cleaning agent, like soap. You spread D-21 on the area, leave it for 2-3 minutes and then vigorously scrub the stains off and then clean the area with fresh water, or at least by a clean wet mop like you would do with soap water. The dirty marks do not magically disappear just by pouring this stuff on them. You need to scrub them out. If necessary repeat it, even twice. You also have to remove all the dirty water, otherwise, it will be reabsorbed back into the floor. You would not leave dirty soap water to dry up in the sun. The final result is not visible until the next day when the floor has completely dried up. With a lot of continuous stains, you should preferably use this once a month.
However, if metal stains have been seeping through the marble for years, like rust stains, that have already coloured the inside of the marble by a chemical process, they can no longer be removed. D-21 cleans the surface only. It cannot penetrate the coloured stone to clean the inside of it.
This is a very good product! Highly recommended! It does what it promises to do, but it does not perform miracles. The only minus point is that you have to use a lot of fresh water to remove the D-21.