How to Remove Grout Film on Non-glazed Porcelain Tile
Sometimes, when using heavily pigmented grout over a non-glazed surface you get a grout film, despite your best efforts to prevent it.
Here I used clean wash water, and I didn't let the grout sit too long on the tile surface and I still got heavy grout film.
I waited four days for the grout to cure. Then, fortunately a mild dilution of phosphoric acid took care of the problem and brought it back to a nice clean surface.