I have a property in Spain that has had new flooring installed. There is (was) no water in the property, so the builders could only clean the tiles sporadically after installation (grouting). To top it off, the tiles are of a cheap variety, so they are quite "porous." The result is that there is now grout on all the tiles, and it has been like this for 6 months. I travel sporadically between Sweden and Spain and have not had time to deal with the problem. Trying with water and a scrubbing brush/sponge will not work. I have been recommended to use a special (Spanish) product and then use a rotary pressure washer like the kind used for decking. Can't find any good images right now, but I think you get what I mean... --- Are there other better methods? Is there a machine that can "sand" the surface and if so, what type?
I have used a polishing machine (the kind for polishing cars/boats) from BilTema and vinegar. Just pour out the vinegar, let it sit for a few minutes, and go over it with the machine a few times. You can feel quite well with your hand when it is "smooth" and the joint is gone.
Just want to add that the tip I received before is to use hydrochloric acid (like Biltema's or the equivalent in Spain) and then possibly a rotating pressure washer and/or brush and water. Gloves and hard-wearing clothes, possibly glasses and a mask. A polishing machine sounds interesting and kinder...