Step 3: Add the grout
Once the glue had cured, we applied the grout. I mixed up a very small batch of grout, mixing it to a fairly stiff consistency. After mixing, I let it sit for about 20 minutes and mixed again to make sure all the dry ingredients were mixed well.
Since I was doing this part by myself, I wasn't able to photograph the process, but it was pretty simple. I placed a bit of the grout mixture on the table and forced it into every gap with a plastic putty knife. After covering all gaps, I then let it sit for about 20 minutes.
Next, I took a damp sponge and begin wiping the grout off the pennies, cleaning the sponge frequently as I progressed. After cleaning all the grout from the tops of the pennies. I wiped them down with a dry paper towel. All that was left was to let the grout cure overnight.