Struggling with black spot
I have recently started my pressure washing business and it has been going well and I have done around 15 jobs or so now.
My main issue is black spot... I can get the surface looking almost perfect but there is always some black spot left behind.
I have recently completed a driveway, and I am going back tomorrow to finish it. (Re sand)
The customer also wanted their patio cleaning which I did at the same time but it doesn't look great and still has a load of algae and black spot left.
My method is, wet all surfaces, apply 1:4 mix of SH agitate and dwell for 15 mins and pressure rinse off but for whatever reason, on the patio the black spot and algae is not moving. For some background information, the patio and concrete area is around 60 years old so I'm wondering if it's just baked in.
Pic is of the driveway I completed still wet, unfortunately no pictures of patio area.
Any help appreciated