Hi Spans,
Dave has given a good answer. Here are my thoughts:
First of all this would appear to be a moisture issue and it is simply that there was still too much moisture in the floor to allow the sealer to cure correctly - you have been a bit unlucky as three hours, can sometimes be enough but in these two jobs there was clearly too much moisture left in the installation.
We also have to be careful not to confuse breathability in a CURED sealer from its tolerance of excess moisture DURING CURING.
All Aqua Mix Sealers (even grout colorant to some degree) are breathable -in that they will allow moisture vapour to dry out through the sealer. However if there is too much moisture present in the stone/tile/grout, and even the underlying bed (all that cleaning moisture has to go somewhere, and old Vic floors often have very absorbent lime-mortar beds that act like a sponge) during the application of the sealer, then it can affect the curing of the sealer, hence it curing milky white, or ‘misty’ as you describe.
It is true that as Seal & Finish Low Sheen is a water based sealer, it is somewhat more tolerant of moisture than some other types, but there is a limit to how much it can cope with, and on these two occasions you obviously exceeded that limit. Also, coating sealers are more susceptible to this effect than impregnators. Believe me, if you had tried the same thing with another well know brand of coating sealer, you would have had a much worse effect, or you would have come across it sooner, on many other jobs.
The moral, if there is one, is that however tolerant the sealer, it is always best practice to leave as long as possible for moisture to dissipate before sealing. We have used this sealer in many similar situations very successfully so, as I said, on these two you have been unlucky and a longer drying time was needed after cleaning, before application of the sealer (or measures to accelerate the drying as Dave suggested).
I doubt the painters did anything, but is is possible that some spilled spirits could have striped the sealer on contact. Sometimes when we strip a sealer and don’t do it thoroughly, or don’t leave the stripper on long enough, wash it off too soon etc, then it only does half a job, which could result in partially breaking down the sealer and this would leave a whitish sealer residue. This is a possibility, but I think excess moisture is the more likely.
To fix it you have to strip, easiest way is to use SCR, neat, 15 to 20 minutes, then add some hand hot water to the floor then scrub with a pad. Then when thouroughly dry, re-apply.
Incidentally, if you are going to do a lot of this, then it could be a good idea to invest in a moisture meter. If you have one, then you could also check the moisture readings of the floor before you clean it, so that you know what the ‘normal’ situation is, it is possible that in some old floors, with no DPC there can be a lot of moisture rising from below in any case ( the floors where you have experienced efflorescence may have shown this)
Hope that was helpful.
Ian