If glazed they won’t need and should not be sealed. If they are porcelain, (probably unglazed) then a temporary coating sealer could help. In this case I would lay the tiles, then seal, then grout, then when the grout has cured use an alkaline cleaner to remove the temprary sealer.

If you wish you can seal the grout later with a grout sealer or if you think the tile is a little bit porous, less lkley, then use an impregnator all over the tile, prior to grouting (so in place of the coating sealer), then when the grout is cured re seal and sea the grout as well.

Hope this helps

Ian

I get so many questions on polished black porcelain so you are not alone.

First of all, did your porcelain have a transit wax, and if so was it removed prior to sealing? If there was and it was not removed, this could have prevented your sealer from penetrating correctly – worth exploring.

Second, the water marks – unfortunately, the whole point of porcelain tiles it to make a ceramic tile with some of the same properties as glass – it is made from similar materials and in a similar way – (large amounts of silica fired at a high temperature makes a ceramic material that is very similar to glass). If you wash glass windows, we do not tend to leave the water to dry of its own accord, if we do we get streaks and marks. Instead we use a squeegee or chamois leather to dry the moisture away. The reason is that no water is 100% pure, it almost always has some impurities in it, (calcium etc). So on highly polished floors (even glazed and polished stone etc) it is always a good idea to dry the floor with an old towel after washing – remove the water before it dries and you won’t have the water marks or streaks. This as I say happens on most polished surfaces but it stands out on black much more – so not a fault but a characteristic of the material.

Regarding the foot marks though, this is more complex and is due to the microscopic texture that is present due to the polishing process of porcelain, my article I have some diagrams that try to explain this.

A sealer may help here, provided it can get in (i.e. no tranist wax) but it may not stop the marks altogether. Is it a falt with the tile? -Very hard to prove this but I do understand your disapointment. The good news is that over time, this issue does seem to lessen somewhat.

Hope this helps
Ian