I'm really struggling with black spot on a York stone patio at the moment, we have tried numerous applications of hypo and it's not touching it, as a last ditch attempt on Friday we sprayed it with algoclear but I'm not confident that it's going to help, so any ideas on how to proceed, I will get some pics tomorrow.
Hi Rob, the dreaded black lichen spots on York stone indeed are a curse, the only tool i have found to remove them completely is to scrub them with stainless steel scouring pads which is a hand and knees job and i know that's not practical but the only way i ever found to completely remove them.
If i can share another method that i have never tried but maybe worth a shot if you are willing, and yes i will get put down quickly for this as most of the guys on this section know about my aversion to using sodium hydroxide based cleaning chemicals, but here goes anyway get some gel oven cleaner which is laced with sodium hydroxide and with a small paint brush dab each of the black lichen spots with the oven gel and allow it to dwell for 5 to 10 minutes, then afterwards see if your pressure washer with a turbo nozzle will rid the little blighters just a thought in case you think its worth a go. Meant to say also Rob that gel oven cleaners it dose not matter which brand as they are all heavily laced with sodium hydroxide and will soften and breakdown heavy baked on carbon from ovens,grilles, after a dwell time of 1 to 2 hours, and because they are so aggressive can even cause chemical staining to stainless steel surfaces, so when it comes to using it on York stone you may have to play around with it as i can't say what kind of reaction you may encounter depending on the amount of dwell time required as i say its a shot maybe worth trying out. Tadgh