I have always sealed after but i have also done very little work with marble tiles esp dark colors esp on a floor.
Carrara marble sealing before grouting.
The edges of the tile are much more absorbent and if the grout color is dark then the dark water from the grout will absorb into the edges of the tile and create a picture frame stain.
I just installed marble tile in an entryway dark green with dark grout.
Because marble is a natural stone and will absorb color you need to seal them before you grout to prevent the grout from discoloring the tile.
I am using a grout from a store i never used before and they told me to seal the marble before grouting because the grout might stain the tile.
Carrara marble needs sealed.
He says it s not necessary to seal the tiles before grouting and that he s done it many times without problems he has 25 yrs experience in stone work however you have previously recommended to me that the tiles be sealed.
I used this sealer and just put it in a spray bottle and sprayed three coats on the ungrouted tile.
Now if your grout is a dark color then it can be advantageous to apply a sealer before grouting to keep the grout color from absorbing into the edges and discoloring the marble.
You ll have to replace those to make it look even.
Sealing before grouting is smart when using a dark grout color with light colored marble tile.
I prefer to seal mine before grouting.
Then i used a grouting sponge to wipe off any excess.
If these deposits are close enough to the surface they can be oxidized by water absorbed during a shower creating a rust stains.