[Ant C]

Hi.
We cleaned the Indian sandstone 6 months ago with a mild hypo mix to remove algae and recently cleaned it thoroughly with low pressure flat surface cleaner to remove the dirt and finished off with mild hypo mix. 4 weeks later and there are a lot of white powdery deposits dotted around.
The patio was newly laid about 18 months ago. We never had any problem after the first clean.
Any ideas or solutions to the unsightly problem would be appreciated. It is a very expensive patio.

[Ant C]

Does nobody have any ideas of the cause or the solution?

[Kev Martin]

Probably efflorescence go over it dry with a low speed mono rotary fitted with a soft to medium brush

[Mike Halliday]

Hypo does leave a white, salty looking residue if not rinsed off, did you leave the hypo on the stone?

Give it a light freshwater rinse to see if that helps.

[Ant C]

We didn't let the hypo dry out and the paving was wet from the pressure washing, but I used a hose to rinse it and might not have rinsed it thoroughly enough, quickly enough. It was a hot day.

If it is the hypo residue, can I just rinse it off or will it keep re-appearing? I read that salt will dissolve back into the stone and then appear when it dries again.

[Kev Martin]

We didn't let the hypo dry out and the paving was wet from the pressure washing, but I used a hose to rinse it and might not have rinsed it thoroughly enough, quickly enough. It was a hot day.

If it is the hypo residue, can I just rinse it off or will it keep re-appearing? I read that salt will dissolve back into the stone and then appear when it dries again.

You read wrong!  It is called efflorescence.  If you wet it again you are feeding it!  Efflorescence needs three things to manifest itself.  Water, salts from anywhere and a Route.  Take anyone of those 3 away and you will get rid of it.  Now all you do is dry brush off any that has formed on the surface to stop it crystallising and hardening into a scale. 

[Kev Martin]

Hypo does leave a white, salty looking residue if not rinsed off, did you leave the hypo on the stone?

Give it a light freshwater rinse to see if that helps.

It is not just Hypo that causes it!  Any chemical will leave a salt residue on stone if you wet it enough and there are enough ground salts for it to form efflorescence!  Even plain water

[Mike Halliday]

Does he know it is efflorescence? At the moment he has a white residue on the surface, if it’s residue from not rinseing (as if often seen  from failing to fully rinse away Hypo especially if used  at a strong dilution) then all it needs is washing off.

If it is efflorescence why did it not appear with previous cleans or through natural wetting from rainfall?  All any of us can do is give a calculated guess

[Ant C]

Thanks for the help.
I popped in recently and hosed it down with a stiff brush and will be going back on monday to see if there's any change.
I have bought some effloresence remover to do test areas if required.

[Kev Martin]

Thanks for the help.
I popped in recently and hosed it down with a stiff brush and will be going back on monday to see if there's any change.
I have bought some effloresence remover to do test areas if required.

So you hosed it down wet and not dry as per my advice?  If it was efflorescence you will have exacerbated the problem   If as Mike says it was excess Hypo you will have got rid of it

[Ant C]

The rinsing with a stiff brush got rid of it.
Thank you very much.