Varnishing natural black slate tiles

Hi. Could be a number of things so i'm going to give advice had i put the floor down so you might be a tad wiser at the end. Although porous, slate only has a certain amount of 'soak' in it. Generally any porous stone has to be sealed before or just after it has been laid. This is always done with a small sponge with minimal sealant and gently at that. You'll know if you are being too vigorous the sealant will start to lather up. Wipe with a dry cloth a few minutes later. I do it before fixing so that if any adhesive gets on it, it'll wipe off without leaving stains or residue. When the slate has been fixed and dry i give it another light coat of sealant, always working away from the area. Also, try to keep off the area for an hour or two as trainer/shoe prints are almost impossible to get out of sealant unless stripped off again! The next process is grouting, the messiest by far. By having 2 amounts of sealant on the slate you now have a barrier for washing the excess grout off the slate. Cleaning is everything when it comes to porous materials, i've had to clean the same area multiple times to makes sure all the grout has been removed. Only when i'm satisfied with that everything is clean and dry will i put the final coat of sealant on. Let's say that grey grout was used and not cleaned off 100%. It has white dye which will show up as a residue which may have then been sealed in! The other reason (and this is shorter) is that as i said earlier, slate only has so much 'soak in' and then after that the sealant will pretty much sit on the surface. This could be what is turning white. Now slate looks great when it has just been cleaned and sealed by will dry out and change colour slightly which is why people do over seal. If you are really unhappy with the finish then there are products which can strip the sealant back. If there is pre-sealant staining then this also can be cleaned. I imagine your slate has a 'riven' feel (not flat) which is tough to clean if too large an area has been grouted in one go. I always thought yacht varnish was for wood and personally i wouldn't put it on my floor if i had slate. If it is a 'look' you are after then you can get matt or gloss sealants which affect the overall look of the slate, either shiny or flat in sheen. Anyway, i hope this has been of some help but if not, seek professional help from a tiler and not just someone who sells tiles in a shop (unless they were a tiler)!! Cheers, Jel

You have use the wrong sealant I suspect. If it's something like Lithofin slate seal its no for wet areas, use Lithofin colour intensifier and you wouldn't have this Whiting problem. Your original sealant needs completely removing before you do anything more.