Somewhere I have a booklet regarding cleaning but can't remember where I purchased it from as it was many moons ago.
One of my bug bears though is that when Sheffield Council did the safety test on them they laid them flat if unstable.....name side down though so even though there are some of my family graves that have stones, unable to read the inscription.
Regarding spraying though would using one of those garden sprayers be ok to use?
Re laying down unsafe gravestones face-down, the cemetery workers responsible are probably DNA matched to the supermarket workers who put massive yellow reduced stickers over important cooking instructions!! The ones you can't peel off easily.