It's new porcelain tile that I just put down. In hindsight, I should have done the grout in small sections, and wiped it off the surface while it was still fairly wet after completing each section. Instead, I grouted the whole 400 sq ft room at one time, and consequently now have week-old dried grout on the surface. I'm finding that scraping it off square inch by square inch with a piece of hardwood and then wiping the surface with a wet rag is actually the best method, as counter-intuitive as that may sound. I'm about 2/3 of the way done. At least the weather is lousy right now, so there isn't a whole lot else I could be doing, at least not outside.
