Black spot removal - Neewbie

[Qwerty24]

Hi all,

Quick thanks to all the advice and guidance I’ve read on this forum over the past few months.

I have a slight issue.
Im very new to jet washing and only trying it as a side project to my other business.

Currently on my 1st job where the slab has required Hypo for black spots, however I don’t feel it’s working like I’ve seen on many videos and threads.

I opted for the 10% Hypo from a local pool shop.
I have tried a 1:4 mix and a 1:3 and scrubbed but still very little change. I tested a spot with just neat Hypo and that doesn’t seem to have done anything all this after 20-30mins for it to work.
I’m applying with a small 5l hand pump spray and some washing up liquid.

I will upload the photos of the before and after stage of my current progress. Any advice would be greatly received.
AJ

Edit: images are currently too large to upload atm

 

[K in Kent]

Personally I always use 15% hypo but I'd srill expect 10% to work at that dilution and definitely neat. A pump up sprayer is fine to use (make sure you wash it through with clean water every time or it won;t last long).
Perhaps it's an old batch of hypo that's lost its potency?
Would seem there must be something wrong with it if it's not working. I'd take it back and complain.
Maybe someone else has got a better idea what the issur could be??

 

[Dave B]

Sounds like hypo is old
Once treated try a quick blast with pressure washer as once it’s hypo’d it should lift easily
Are you scrubbing it in after applying?

 

[THL4KEL]

Just use it neat if it's not coming off. I use 10 percent all the time.