Over the years I've heard from several folks that they had neglected to find out ahead of time if the cemetery they planned on working in had clean water available on site. Water is the most important item needed for cleaning a gravestone, and you need a lot of it. How much? I would suggest a minimum of one gallon per cubic foot of gravestone. So, if you plan on cleaning a 3' wide x 3' high x 1' deep grave marker, I would have nine gallons of water. Again, that's just the bare minimum. More water is better. Keep the stone wet while you clean.