I used the shop walls of my mom and dads pet shop for this little experiment. They didn't do any kind of repainting when they moved in and had inherited these little circles all over the wall. Which, as a side note, is the reason that you can NOT hold me accountable for the less than perfect circles. I only colored inside the lines with my homemade chalkboard paint.
1. ALL of the dry non-sanded grout at the hardware store made a point of calling out that it was "dry non-sanded grout with polymer." Uh, polymer? Did that matter? Nope not a bit.
2. The recipe that I pinned said to make sure there are no lumps in your paint/grout mixture. It doesn't take but a minute to blend up a small batch and remove the big clumps. But your mixture will look grainy and that's okay. When you start painting you'll see that grainy paint smooth out under the weight of your brush...which by the way, I recommend a foam brush so that you won't see brushstrokes.
Pinterest and search "chalkboard paint"...you won't be disappointed!