Limestones and all other natural stones are absorbent in nature and absorb resources very easily, causing the surface to stain. The best removal plan for Limestones is the stain turnaround process using Poultice. Poultice is made from powdered whiting and hydrogen peroxide or a chemical reducing agent. The process is very simple and the type of poultice depends on the nature of stain but you can easily make it at home. Here is a link that has a step by step guide on how poultice is made and applied for stain removal.