Concrete Block and Masonry Painting

Masonry Is Not Drywall: Why It Needs a Different Approach

Concrete and block are porous and alkaline, and they move water constantly. Rain soaks in, ground moisture wicks up, and the sun drives it back out. Any coating has to cope with that traffic. Paint chosen for a smooth, dry interior wall will trap vapor, lose adhesion and peel off in sheets. That is the core reason concrete and masonry painting is treated as its own specialty rather than an afterthought on an exterior bid.

In North Central Florida most commercial buildings and a great many homes are built from concrete masonry units (CMU), so this comes up on garden walls, warehouses, church fellowship halls, pool houses, foundations, retaining walls and the block interiors of shops and garages.

Reading the Wall Before Choosing a Coating

The first visit is diagnostic. We look for white crystalline deposits, called efflorescence, which tell us salts are migrating out with moisture. We check for hollow-sounding areas, spalled faces, cracked mortar joints and rust stains bleeding from embedded steel. On previously painted walls we look for blistering and test adhesion. On new pours or recent block we ask how long the masonry has cured, since fresh concrete is highly alkaline and can damage some coatings.

Where moisture is in question, a simple plastic-sheet test taped to the wall overnight shows whether vapor is condensing behind it. That result, plus the location of the wall (below grade, against soil, shaded, or in full sun) drives which system we recommend.

Prep Steps That Make the Coating Last

Skipping block filler is one of the most common shortcuts we see on painted CMU. Without it, a wall can look fine from the street but have thousands of tiny pinholes where water gets behind the film.

• Clean with pressure washing and a mildewcide where growth is present
• Remove efflorescence mechanically and neutralize, rather than painting over it
• Rake out and repoint failed mortar joints; patch spalls with a compatible masonry repair material
• Route and fill cracks with an elastomeric or masonry-rated sealant
• Apply block filler on rough, open-textured CMU to close pinholes and create an even base
• Prime chalky or previously painted masonry with a bonding or masonry primer

Coating Options for Different Masonry Surfaces

For above-grade exterior block and stucco, an elastomeric coating or a high-quality 100% acrylic masonry paint is usually the right topcoat. Elastomerics bridge hairline cracks and shed wind-driven rain, while acrylics breathe more freely and suit walls that need to dry out. Brick that has never been painted deserves a pause: once painted, it is very hard to return to bare, so we sometimes suggest a breathable mineral-type finish or a clear water repellent instead.

Interior block in shops, gyms and utility rooms usually gets block filler and a durable acrylic or epoxy-modified finish that can be washed. Concrete floors and slabs are a separate category with their own prep; see garage floor epoxy coating. Below-grade or water-exposed walls may call for a dedicated waterproofing system, described on our waterproofing and protective coatings page.

Timing and Weather on Masonry Jobs

Masonry holds water longer than wood or siding. After washing, or after a run of summer storms, block can stay damp inside for a day or more. We plan dry time into the schedule and check surface moisture before coating. Very hot direct sun can also flash-dry thick coatings, so on large south and west walls we work with the shade as the day moves.

Rural properties add one more wrinkle: lime-rock dust from unpaved roads settles on walls and acts like a bond breaker if it isn't washed off. Around Williston, Trenton and the outskirts of Newberry, we pay extra attention to rinsing the lower wall and any surfaces facing a dirt drive.