[ross12345]

how do i remove the blck spots on the patio slabs please iv preshured washed em but cant remove the black spots

[Griffus]

There are occasions when certain marks become absorbed into the slabs thus becoming part of them.

No amount of pressure will remove these. You will damage the slab if you try.

I always point this out before starting work so the customer is not left disappointed.

[drivewasher]

The black spots are lichean, a ytpe of fungus. Pressure washing does get the surface fungus off but it has a kind of root syrtem that grows into the fabric of the surface it's on, this is what you can still see. looks like black spit or oil marks.
There is a chemiacal available to kill it but it takes a few days, then it can be removed by pressure washing.

[drive surgeon]

lichen remover 

[drivewasher]

Go on then J show us!

[Ged]

wet the area then spray the spots with a little dilute hydrochloric acid (brick acid) , dwell for 10/15 minutes,this will soften the lichen just enough to shift it with a turbo lance. take sensible precautions when using the acid and wear protection, wash the area down with plenty of water and  be aware where the run off is going.

Ged

[collins ReVive Stoneworks]

well done ged

did that on my own slabs acid is great for most things

pressure washing alone seems to encourage growth after and very quickly

so does anyone have an after treatment???

[BDCS]

Lichenite