Also, it is always the best practice to read the entire label of every product that you intend to use. In this situation, a mixture of several mistakes led the floor to be in that miserable state. The floor was clearly beyond an easy fix, therefore I had to strip the old sealer off the entire floor. This process is never easy on a riven surface and caused some difficulties here too.
In the end, it took nearly twice as long to completely remove the old coating than expected, but it did worth it.
Once the floor was clean I neutralised it again to get rid of any residue which could upset the sealer. After all this, I left the slate floor to dry overnight.
On my return the next day the floor was completely dry so I could start applying the new sealer. The customer decided to go with a topical sealer to make the floor easier to maintain. So I applied four coats of a good quality water-based topical sealer which gave the floor a nice sheen too.
As you can see in the pictures below I managed to make quite a transformation. My customer was over the moon with the results.
