OK, it sounds as though you have got a riven finish – a natural finish where the slate is left as it is when it is split. This can be very smooth but with the occasional bump or it can be an all over texture. This variation oin the surface could well be naturally varied in colour also. It sounds to me as if you had wanted a fine rubbed (honed) finish – where the slate is ground to a very smmoth and flat surface. I would double check this with your supplier. You could still have patches of darker or ligher colour in it though.

Hope this helps

Ian

Sounds like stain of some kind, if washing up liquid does not help, try an alkaline cleaner, you could also try a poultice

hope this helps

Ian

First of all though, before you use an acidic cleaner (I assume you meant for the grout residue?) you will need to remove the remaining Lithofin Slate Seal. You will need a stripper for that, you could try Sealer & Coating Remover, or try whatever product Lithofin recommend for its removal. Then you can follow the advice for removeing any grout residue (although you may find some of it coming away as part of the stripping process)

OK, Q1. Oops, I hink I answered that above

Q2. Yes, to my knowledge it is a very good sealer. Is it the best? – hard to say. I can recommend an alternative in Seal & Finish Low Sheen (available via All for Stone) is it harder wearing than Lithofin- I don’t really know, maybe, perhaps just a bit, but the main differenence is it is water based and much nicer to apply. But they are both topical sealers and will therefore wear and so in a high traffic area they will need frequent reapplication, I think that the 3 years you have had the Lithfin on for is the very max limit for this type of sealer. You may be better reapplying every year.

Q3. You pretty much answered this yourself, keep grit off the floor (dust mats) sweep vacuum regularly, Mop up isolated spillages as teh occur, use a mild eutral cleaner for every day or routine washing. Avoid using harsh cleaners. Alkaline cleaners will remove some coating sealers or at least dull them severely.

Q4. As often as required. I cannot speak for the Lithofin product but the Seal and Finish Low Sheen can be applied to itself wiothout theneed for stripping back. As you are going to have to strip the floor anyway there should be no problem putting any coating sealer down.

Q5. For outside I would use a good impregnating sealer, the external environment is too harsh for a coating sealer.

hope this helps

Ian