I thought that is one of life's problems solved.

Until I saw the price............
www.thepatioblackspotremoval.com

 

The site isn't functioning @roders.

 

http://www.patioblackspotshop.com/for-natural-stone-10-c.asp
This works out at a heady £20 a litre. Costs the same as cleaning your patio with neat Bombay Sapphire Gin !!

 

There must be cheaper ways than that to clean stone.

 

What a waste of Gin

 

Not to my knowledge ,those black spots that have taken years to develop just can't be removed ,I have tried acid and everything else.
Just waiting for someone to come up with a cheaper formula.............
Anyone?

 

I wonder if @Zigs can come up with anything?

 

You want Zigs playing around with chemicals !
@roders , you have tried brick acid ? I have some brown stains on my indian stone slabs. Its liquid fertiliser spills caused by yours truly . I find a good wire brushing cleans that up . Black spots I presume are a type of deep rooted lichen ?

 

You could try polticing on them, you'd need the stone to be dry, then wet the spots thoroughly with a strong bleach solution, preferably hot.

Let that soak in and apply a flour and water paste, cover it to keep rain and birds off, but not so that it won't dry out.

As it dries it should draw the staining out, you just scrape it off and dispose of it afterwards.

Learn that at college for removing stains from buildings, but found it more useful for removing red food dye that the kids spilt on my solid beech worktop

 

Oh, don't get bleach anywhere near brick acid as you'll make mustard gas and die of death

 

Nice little top tip from Zigs , for any of our homicidal lunatic members