I've used hypo a few times in the past on block paving and paving slabs with good results. Got a job to do this weekend which is all block paving apart from a centre piece that looks like this below. It is borderline line black with dirt and has lichen all over. I wished I'd got a photograph of it because the stone could be different. Basically what I'm trying to ask, is there any types of stone that I need to avoid using hypo on?