We use to make spray bars out of 3/4 in PVC. Cut them to lengths you need, get some tees glue them up. Add spray nozzles to the tees. Cap one end , hose threads on the other. Hang them from scaffold or whatever you make and move quickly. Connect Waterhouse and let them mist the stone. Should open up the pores,
You can get a filter set up with a timer if needed. After they were misted for a reasonable time , I would use a laundry detergent diluted with water a scrub. Finish it off with low pressure , hot wash. Cleaned a whole lot of limestone ballasts like that. Nothing to aggressive as to not damage the stone.