An example of how the PAINTING Guy paint a concrete fireplace.
I usually use an airless sprayer. This method gets the paint further into all the cracks. Once you have your first coat of paint on, you can see all the cracks better.
I use DAP 230 to caulk all the seams, then allow 24 hours or more for the caulking to dry. Once the caulking is dry, I apply additional coats of paint until satisfied with the results.