mosaic studio owner (remember the post about the mosaic?), Peter found out how to clean dirty grout: with toothbrush and citric acid. This really tedious work consists of applying the acid with a toothbrush, then removing it so the acid "eats" the dirt on the surface but not the grout itself, then apply sealer, leave it to dry and apply a second layer of sealer.
OK, so now the grout was clean, but with holes in it (the construction workers did a really sloppy job). He marked the holes with dark pieces of tape, cleaned them, filled them with new grout (this reminded me of filling dental cavities), removed the surplus from the tiles and applied 2 layers of sealer. Crazy work. I helped a bit with the sealer, Tom helped more. This went on during a few Saturdays.
If we'd knew then (when the tiles were laid), what we know now (that there is a method of laying the tiles very close to each other, without grout), we could have saved all this. Why is it that you don't get experience until just after you need it?