[simonr]

i've used weak hypo before & washed it off tarmac ok but i don't think weak will work on these marks so i'm a bit stumped as there's not much drainage  there

[BDCS]

Hypo 5:1 with plenty of amine put on with a wfp will do that but as I said on facebook check the parapet gutters as that may be the cause. Those porous coping stone are notorious for soaking up the water and making those stains. You could also use quats if you wanted as its a safer bet

[simonr]

Lauryl Dimethylamine Oxide,  is that what you mean by amine
only got a 19 rung double ladder now, not sure if that will get me up to those gutters be interesting to try though 

[BDCS]

Its just a soap / thickener and allows the hypo to cling and work. Apply it with a wfp  - it should come off easily. Algo clear is a safer option though

[simonr]

ah right,  i used fairy liquid  when i did my house

[Smurf]

If you mix hypo with any kind of soap and/or proprietary off the self cleaner you need to be mindful as if it contains a certain chem in it can trigger a chemical reaction and give off a lethal poisonous gas. I will tell you what that chem is if/when you sign up to my £600 softwash trianing course 

[simonr]

If you mix hypo with any kind of soap and/or proprietary off the self cleaner you need to be mindful as if it contains a certain chem in it can trigger a chemical reaction and give off a lethal poisonous gas. I will tell you what that chem is if/when you sign up to my £600 softwash trianing course 

ha ha !!  though to be honest if i had a spare grand i would risk the dark streets of luton for a chance to get a lot of easily accessable knowledge in 1 place
i know there's lots on here & i've been reading your (i think) posts on another forum but its deciding who's posts you takes notice of & making notes etc & its making my eyes & brain bleed