Getting the Finish Right on Burnished CMUWords: Block Design CollectivePhotos: Kehm ContractorsWhen Good Work Goes AwryThe wall is complete. The lines are crisp. The coursing is tight, and the burnished CMU delivers exactly the refined appearance the design team envisioned. After weeks of coordination and careful installation, the project is finally approaching the finish line.Then comes cleaning.What is often treated as a routine closeout step can become one of the most consequential. In some cases, the very process intended to improve the wall’s appearance can end up diminishing it instead. The mortar joints can become uneven, the surface can lose its signature sheen, and staining and efflorescence can disrupt the palette.Phil Ledent, PE, SE, Executive Director of the Masonry Institute of Michigan, has seen this issue arise on several projects. Burnished CMU installations that were otherwise well executed were compromised during final cleaning. “Nothing undermines the finished appearance of a masonry wall faster than improper cleaning,” he says.As an engineer, Ledent typically reviews outcomes rather than directing means and methods. Still, it is a pattern worth calling out, if only to help other projects avoid running into the same issue. For mason contractor Shawn Kolterman of Kehm Contractors in Omaha, Nebraska, the problem rarely stems from a single decision. It builds over time. Choices made during design and installation all influence what happens during cleaning.During design and material selection phases, a colored mortar may be chosen to complement burnished CMU. The goal is understandable, but consistency in the finished appearance can be influenced by a range of factors. Exposure to sunlight, temperature, humidity, and airflow all play a role in how mortar cures. Finishing techniques like strike timing and tooling pressure can also contribute to variation, with joints typically producing the most consistent results when tooled at a thumbprint-hard stage using the appropriate jointers.When those variations show up, crews may look to cleaning to even things out, which is where issues can begin to compound.Bob Holmes of HolmesCo Inc. notes that the more that can be addressed during installation, the less there is to correct later. “If you’re knocking down clods and wiping smears as you go, you’re left with a light film instead of something you have to fight later,” he says.That early attention helps maintain a more consistent appearance and reduces the need for abrasive cleaning. In practice, that often means slowing the pace slightly to keep the wall clean as it goes up.That same mindset extends beyond the wall itself. Establishing a project mockup allows the design team and owner to align on expectations for workmanship, joint tooling, cleaning, and final sealer appearance before full production begins. While mockups can feel like an extra step, they provide a controlled setting to review the complete system and avoid surprises later in the project.Cleaning Methods Require Control“Use the least aggressive chemical necessary to achieve the desired results.”— Historic masonry preservation guidance (U.S. National Park Service)Holmes says that the same principle applies to new construction as well. “Burnished block is an acid-sensitive surface. The more aggressive you get, the more likely you are to create problems.”High-pressure washing can introduce problems by pushing solutions deeper into a porous wall surface. When the detergent is not fully rinsed out, both the unit and the mortar joint can be affected, compromising appearance and even performance over time.“We prefer the old-school, bucket-and-brush method of cleaning burnished CMU,” Kolterman says. “It takes a little more time, but it allows for a more controlled process and a more consistent finished appearance.” That approach reflects a broader way of thinking about the craft. “We are building something that ought to last for 50 years or more,” he adds. “First cost always comes up, but this is one area where a little more care upfront can make a significant difference in the long run.”It’s not just a question of speed versus quality. It’s about how force is applied. Kolterman offers a simple comparison: “Think of it like doing dishes at home. Residue that is easily removed with minimal pressure using a sponge often won’t come off in the dishwasher. Once it goes through the drying cycle, it ends up bonded to the plate, requiring significantly more force to get it clean, a force or pressure that may scratch or damage the surface.”Cleaning methods matter, and calibrating them to specific materials is essential. The concentration of cleaning agents for burnished or ground-face CMU is typically much lower than for clay brick because the materials respond differently. Using the same solution across different conditions can introduce risk, particularly when paired with high-pressure washing. Mortar type, pigments, the substrate, and adjacent materials all factor into how the wall responds. What works for one condition does not automatically translate to another, which is why the detergent needs to be tailored to each job.Following manufacturer guidance is also critical. Block producers often provide specific recommendations for cleaning agents, application methods, and compatible sealers based on the desired finish, whether a penetrating treatment or a higher-gloss appearance. Aligning with those recommendations, even when it requires coordination or substitution approvals, helps reduce variability and avoid unintended outcomes.Using an adequate volume of water is especially important when pre-wetting and thoroughly rinsing the wall. Holmes says a good rule of thumb is five gallons per minute. “At our Maypearl Brick Academy in Texas, I always ask masons: ‘Would you ever wash your shiny black truck in the hot sun with a pump sprayer and expect a clean finish?’”Without enough water to properly pre-wet and flush the surface, residue can be left behind, leading to streaking and inconsistent results. The same principle applies here.Interior applications add another layer of complexity. Once doors, lockers, flooring, and surrounding finishes are installed, cleaning options become extremely limited.There’s less margin for error, so installation must be precise. Holmes advises interior crews to slow down and treat the wall as a finished surface from the outset. What might be correctable on an exterior wall becomes far more difficult once the building is closed in.Coordination Before Closeout MattersConstruction schedules and patience often compete. Mortar can take 60 days or more to fully cure. During this time, natural variations fade as moisture evaporates and the final color settles in, creating a more uniform appearance. Schedules, however, often require burnished block to be washed and sealed before that can happen fully, yet the wash-down process adds even more moisture back into the materials. Applying an acrylic sealer is the final step. Since that acrylic doesn’t breathe, affording sufficient cure time is critical before locking in the appearance of the block and mortar. Kolterman recommends a team walk-through to review the work before the application. “That’s the time to make adjustments if something doesn’t look right,” he says. Additional cleaning or mortar staining in extreme cases can help improve consistency if needed.“You’ll never regret taking the time and care to get it right,” Kolterman concludes. “Burnished block is a durable material with a unique aesthetic. When done well, nothing else delivers a structural wall system with a granite-like finish quite like it.”Taking these steps helps ensure a consistent, well-executed finish that holds up over time.—As questions arise, there are resources available. The Block Design Collective (BDC) offers custom support to contractors and designers on all aspects of performance, including aesthetics. The BDC is fully funded and available at no cost or obligation, helping teams think through decisions early and avoid problems later.About: Technical Talks
