How to clean sandstone?
I believe you need acid to clean it.Make sure the area you are cleaning,a small bit at the time is wet with water before you put the acid on it.That will save you having drop spots later.Keep it wet with more water for about 5 minutes and then rinse with generous amounts of water.When it is dry keep all traffic off it as it becomes porous and easy to stain again.When you finished this washing,and when it is all bone dry carefully seal it with an apropriate sealer.In England they have many products for this.Look at the websites of Lithofin and/or HG to find what would be best.
