Sealing slate floor tiles
Our recommendations would be to give your slate tiles a coat of sealer prior to grouting but your floor must be dust and dirt free before you give the slate a first light coat of sealer.
Once you have grouted your slate tiles then we recommend another light coat of sealer. It is always advisable whichever stone you choose to do a water test when the sealer is dried which can take up to 48 hours to be fully dried.
A water test involves filling an egg cup with clean water then pouring this onto one part of your dried slate flooring and then should be left for 2 hours. If when you return the water has dispelled into the stone then you need to apply a further light coat of sealer.
If after sealing the slate floor tiles this has been done correctly then you will be left with a pooled group of water sitting on the surface, the same as you would see droplets on a window when it's raining.