The spots should be cleanable, try water first. If that doesn't remove them use an alkaline solution. Here is a cut-n-paste from below:
We are going to remove the stain with an alkaline solution of caustic soda and water. Mix it 10% caustic soda to 90% water. You can get this stuff at any swimming pool supply as it is what they put in the swimming pool when the acid level is too high. The first thing I would do is treat the spots with an alkaline solution to make sure all the acid is sucked out of the stone. You can use either caustic soda and water solution, or if it is a small area and you have Drano, use that. Drano is a great alkaline solution, I don't recommend it for large areas as it is a lot more expensive than the caustic soda. Put the solution on your stone and lay a paper towel over it to keep it moist for 30-60 minutes, then clean the stone thoroughly with water.
When you are finished with thecleaning you should seal the stone with Hydrex, a mineral based sealer by Tenax. Wipe on the Hydrex sealer to impregnate the stone. This will stainproof it and prevent citric acid (and other acids) from etching it. It is a mineral based sealer so it will last a long time.
http://www.defusco.com/tenax-impregnator-and-sealer-liter-p-2095.htmlIf you have any questions please call me at 800-289-6834 during the day.
Glad to be of service,
Mark
