If you plan to paint fireplace stones, there are a pair of important factors you will need to consider before you begin. First, because stone is ill-suited for paint adhesion, you must condition the surface by applying a primer, or the paint will peel from the fireplace soon after application. In addition, because fireplaces are often marred with smoke stains, you must ensure that the primer you choose is formulated to block stains, or the smoke stains will reappear over time.
Scrub the fireplace stones clean using a water-based degreaser. Rinse the stones using damp rags. Allow the stones to dry.
Cover areas of the fireplace you do not want painted with painter's masking tape. Protect the floor beneath the stones by covering it with drop cloths.
Apply a coat of acrylic latex primer to the fireplace stones using a 3- to 4-inch paintbrush manufactured for latex paints. Do not use a paintbrush manufactured for oil-based paint, or the brush will ruin. Work primer into the mortar between the stones by dabbing it with the brush. Allow the primer to dry for two to three hours.
Wash the paintbrush with water.
Apply a coat of acrylic latex paint to the primed fireplace stones in the same way you applied the primer. Allow the paint to dry for two hours, then apply an additional coat if you can still see the primer showing through.