A few years ago we did this variegared bluestone patio, set in concrete. Recently the owner showed me these mold spots. He mentioned that he has scrubbed the patio with bleach and they will not go away. Does anyone know why these may have appeared and how to get rid of?
Thank you!
Replies
If he scrubbed with bleach and they won't go away then they're not mold. Probably some chemical reaction, or maybe just staining of some sort.
Try oxalic acid (from a paint store). If it's iron staining (which looks likely) then that should clear it up.
Thank you for your feedback. This patio was done about 7 years ago and the spots have just popped up within the last year. We will look into oxalic acid.
Did the oxalic acid work? I've used both oxalic and muriatic acid to remove rust stains on hardscape surfaces over the years, but Iron stains typically present as orange on stone surfaces due to the oxidation, while they appear black on woody surfaces. A black iron stain on stone has me perplexed. Trying to decide which product to purchase to clean spots on bluestone that look exactly like the photo in the original post.
I'm guessing that the stains are black lichen, which have deep roots and are therefore tough to kill. There are a number of products that kill black lichen without killing nearby vegetation. Search on-line for black lichen killing products designed for patios.