The stain is from what?

The contractors use tiles cleaner to clean. It is actually diluted HCl (hydrochloride acid) which can easily be bought in hardware store. I think mostly because to clean cement used in grouting and also used to remove stain from welding (dissolved iron stain).

What kind of tiles? My floor tile even applied 3 times also still okay. The only precaution is you need the place well ventilated to avoid suffocation. It will make stainless steel turns black though but it can be polished to shine easily.