Help please with slate patio. Many problems

[homeimprovement]

Hi


We recently had a Bradstone natural slate patio installed by builders. They grouted/laid in the rain.We had lots of cement overspill on face and sides. They promised to clean next day but didn't turn up for 7 days. They scrubbed with brushes/water, to no avail. They used Grimex to no avail, they used ltp grout cleaner/hydrocholic acid which worked but changed colour to light grey. We didn't like the colour so experimented with sealer on 2 slates. Looked good, so we sealed the rest. Done so we thought when they were completely dry. Next day we had efflorescence. Removed that with stainless steel wire wool, as advised by a reputable builder/tiler. This worked but returned slate to the state it was when cleaned with acid.


Builders now want to use ltp colour intensifier and stainblock.

1. We told them we never wanted to use sealer in the first place but wanted the slate natural.
2. We didn't want them to use acid but they said they had no option.

Do we allow them to seal with above product. We're really scared we'll end up with the efflorescence again especially as the weather so poor at the moment ? Also how long does this product last for and do we have to strip and renew every few years ?


Any suggestions please. Thanks for any replies.