Cut-face concrete blocks, also known as “split-face” blocks, are used to apply a decorative finish resembling rough-hewn stone to inside and outside feature walls. However, if you wish to transform the bland gray color to a finish resembling natural stone, simply apply an acid-etch stain. These stains consist of water diluted hydrochloric acid containing acid-soluble metallic salts, and they impart subtle earth tones to the finished wall. In addition, if you prefer a more vibrant color, you can apply a second coat of a suitable concrete dye to add the desired finishing touch.
Spray a little water onto the surface in different locations to see if the wall absorbs water. If the water beads and runs off without penetrating, use a high-pressure power washer to remove any previously applied sealants, curing agents, caulk or paint from the surface. Allow the wall to partially dry; if the surface looks patchy, clean the wall a second and then a third time if necessary. Follow all safety precautions recommended by the manufacturer.
Use a 1-inch paintbrush to dab undiluted acid concrete stain on a small patch near the bottom of the wall to test the finished color. The acid content will etch the surface and allow the metallic salts to penetrate. Allow time for the test area to dry thoroughly; if the finish is too dark, dilute the mixture with clean water, apply a second test area, and keep diluting the mixture until you achieve the desired finish.
Mask off adjacent walls, ceiling and floors with blue waterproof masking tape and plastic sheeting where necessary. If you are working inside, open all windows and doors to provide adequate ventilation.
Wear old clothing and a long-sleeved shirt, together with safety goggles and rubber gloves, before starting work. Use a sprayer rated for acid and with no metal parts, and fitted with a circular spray nozzle.
Insert the sprayer feed tube into the plastic bucket containing the acid-etch concrete stain, start the sprayer, and apply the first coat using a left-to-right and then right-to-left motion. Have an assistant work the wet stain into the wall by scrubbing the surface lightly with a medium-bristle brush, using a circular motion.
Apply a second light pass with the sprayer just behind the scrubber before the solution dries. This will remove any remaining brush or sprayer marks. Continue this way until the entire wall is covered. Allow adequate drying time before inspecting the finish.
Test and dilute a finishing coat of concrete dye as outlined in Step 2 if applicable. Insert the sprayer feed tube into a bucket of clean water and run the sprayer for two or three minutes to clean out the machine. Apply a second coat of dye using the same technique described in Steps 5 and 6.