Suggestions please for black spot removal patio

[blueeyez]

I've got a serious issue with Black spots on the patio and need something fairly robust to deal with it. Ive tried various over the counter things from DIY stores but think I may need something more professional, any suggestions please, I'd like to do it myself if I can.
Regards.

 

[Munroast]

I have been battling this stuff for years, it's pretty stubborn stuff, two options,
opt 1/ turbo jet power washer then Hydrochloric acid (150mm per 3x2 slab) then another blast with the turbo jet - takes about 15 minutes work per slab

opt 2/ the single pencil jet on the power washer, mind numbingly slow job - takes about 15 minutes work per slab

opt 3/ ammonia nitrate. but then you will need a new patio. and house, and village.

 

[Andydevon]

Similar problem with Indian Sandstone. Pressure wash cleaned up the slabs but left the black spots. I used this to remove;

http://www.molevalleyfarmers.com/mvf/store/products/deosan-red-label-hypochlorite

Plenty of agricultural supplies firms sell it as well.

It's just over 10% strength bleach so more than twice the strength of domestos etc. brush or spray on and leave for 5 - 10 minutes then hose off. Don't get it anywhere you don't want cleaning including your sunday best as it will bleach anything. I won't bother pressure washing again just use this next time. If you do brush on be careful not to spray via the brush onto fencing unless you like the covered in little white spots look.

You can get stronger but when you see this work it's not necessary.