Slate Patio efflorescence

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Hi
Had a Slate patio proff installed and they did put a backing sealer on the rear of the tiles to stop damp coming through esp as its around a koi pond .

I have looked into sealers and have purchased lithofin stainstop plus which is oil repellant and is a colour enhancer . While cleaning it down today as its been installed around 4 to 5 months now a couple of white marks have appeared which i put down as effloresence although im not sure as they seem to have appeared also where we have the rattan patio set and i cant say im 100% certain they have appeared after wetting the patio down or they were there before .

But my question is this , can i put the sealer over the top of these and i believe this sealer is breathable so if it is efflorescence it might go naturally in time or do i have to tackle this problem now before sealing .

If i have to tackle the stains now then whats the best thing to use as i have read alot of conflicting stuff on the old tinternet as acid is bad for slate then ive read using a combination of lemon juice, bicarb , vinegar works a treat but these are all acids . So how do i go about getting it off slate .

Please dont reply unless you have tackled slate patios as other types of stone you can use different products on but i dont want to chance using something that destroys the slabs .

Please see attached pics of stains

Are they effloresencence or something else they are more or less flate to the surface i cant brush it off , tried with a paper towel . When its wet it does disappear but then comes straight back , but why are they both round in shape and nowhere else has anything like these stains .

Thanks

Alan

 

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It was laid on a full mortar bed with a backing sealant put on each tile , i know as i asked and watched them doing it , pretty sure they used sbr bond .