what is this slab? looks natural stone

ashe

Albermarle said:

ashe said:

Sodium hydrochlorite. 

It's nasty stuff, you mix it 3:1 or 4:1 with water (add the hydro to water. Not the other way around in case it splashes)

I wore gloves, a proper mask not a diy dust mask, full outfit no exposed skin and sealed goggles. 

Did the job in about 30 minutes then rinsed 

mask did a very good job; as soon as I took it off it smelled like I was in a pool but about 5x stronger 😂

much less effort then pressure washing - got rid of black spot and algae and general muck, but it's obviously a strong chemical 

Presumably you have to rinse it off at the end, and any run off going on the lawn or plants will kill them ?

Yeah you do, I've got a lovely green striped lawn and it's still lovely and green (well, as green as a scarified lawn is!) Nothing died, we moved planters off the patio beforehand. 

10L cost me about £20 delivered, could have got away with less but I needed it quickly so had to go with that.  I paid £35 for a decent mask, cheaper online though. already had gloves and goggles (sealed ones so no spray could get in) 

once it's on a stiff brush is good as you can agitate the lichen as it can be stubborn, I'd say take great care with it but if you do it's a real time and effort saver. 

ashe

Here is the before and after, it had already been pressure washed, and I only discovered at the end that agitating helps; so it would have been a lot better had I done that as I went. Just a few sweeps left and right over the big spots has them come away, I'll probably give it another go next warm windless day to finish it off. But these tired concrete slabs are so bright now, the photo on a cloudy day from a security cam doesn't do it justice! (They're almost 24h apart hence the effects of scarifying more apparently in second pic) 

My lawn doesn't look great there btw as I had just scarified it ready for a re-seed, but you can see it didn't kill my lawn off. 

We also sprayed it on the cobbled stones on the right - it brought them back to life and got rid of algae etc on those 

cherry2017

ashe said:

Here is the before and after, it had already been pressure washed, and I only discovered at the end that agitating helps; so it would have been a lot better had I done that as I went. Just a few sweeps left and right over the big spots has them come away, I'll probably give it another go next warm windless day to finish it off. But these tired concrete slabs are so bright now, the photo on a cloudy day from a security cam doesn't do it justice! (They're almost 24h apart hence the effects of scarifying more apparently in second pic) 

My lawn doesn't look great there btw as I had just scarified it ready for a re-seed, but you can see it didn't kill my lawn off. 

We also sprayed it on the cobbled stones on the right - it brought them back to life and got rid of algae etc on those 

Well done, I will consider the chemical clean, is it acidic chemicals?

Bookworm105

cherry2017 said:

ashe said:

Here is the before and after, it had already been pressure washed, and I only discovered at the end that agitating helps; so it would have been a lot better had I done that as I went. Just a few sweeps left and right over the big spots has them come away, I'll probably give it another go next warm windless day to finish it off. But these tired concrete slabs are so bright now, the photo on a cloudy day from a security cam doesn't do it justice! (They're almost 24h apart hence the effects of scarifying more apparently in second pic) 

My lawn doesn't look great there btw as I had just scarified it ready for a re-seed, but you can see it didn't kill my lawn off. 

We also sprayed it on the cobbled stones on the right - it brought them back to life and got rid of algae etc on those 

Well done, I will consider the chemical clean, is it acidic chemicals?

google?
Sodium hypochlorite is an alkaline
acid does nothing to black spot but may damage certain types of natural stone or concrete/cement .