In this case, the grout was actually vinyl spackling we had leftover from a home improvement project. It was pretty dried out, so I mixed it with water until it was gloopy. Then I spread it over the the fireplace, making sure to squish it into the spaces between the stones. I wiped off the excess with a damp rag and let it dry.
There was still some grout left on the stones. I used a wet brush and another damp rag to remove as much of that as I could. This is why I sealed the stones twice! It took a lot rubbing to remove that extra bit of grout!
And of course, not all of it was actually removable. I used paint to touch up some of the most persistent bits so they wouldn't be obvious. I also used thin washes of black paint to darken the inside of the fireplace.
The grout was a little bit crumbly and fragile, so I sealed the entire thing with another coat of Mod Podge.
While that was drying, I cut a piece of craft wood for the mantle and painted it with thin washes of brown and black paint. I sealed it with Mod Podge and let it dry.
Then - finally - I glued the mantle to the top of the fireplace.
It needs logs and a fire element, plus I want a tiny Seth Thomas clock to put on the mantle. Still, I'm calling this done, at least for now.
This was a fun project. I'm glad I finally got around to making it, and I can't wait to use it in some Winter Fun photos.