The surface of an unpainted concrete masonry block wall presents a challenge that most exterior paints are not designed to handle without preparation. The face of a standard 8-inch block is not smooth. It is porous and irregular, with surface voids from the manufacturing process and the mortar joints creating a texture that swallows paint, produces uneven coverage, and leaves the final finish looking thin and patchy unless the voids are filled before topcoating. Getting a durable, uniform result on exterior block requires addressing that surface porosity directly before a single gallon of finish coat goes on.

Filling Cracks and Holes in Exterior Block Before Painting

Block walls accumulate two types of damage that must be resolved before painting: structural cracks through the block face or body, and mortar joint deterioration at the seams between blocks. Confusing the two matters because each requires a different repair material.

Hairline cracks in the block face itself, up to about one-eighth inch wide, can be filled with a flexible masonry caulk such as Sashco Big Stretch. Big Stretch is a 100 percent acrylic sealant that stretches more than 500 percent without cracking, handles service temperatures from negative 30 to 250 degrees Fahrenheit, and bonds to the masonry surface without requiring backer rod for thin cracks. It is paintable with latex topcoat within four hours of application. For wider cracks with active movement potential, insert a backer rod before caulking to avoid three-sided adhesion, which prevents the caulk from stretching and causes failure at the edges.

Active water leaks through block walls are a different problem. Water actively pushing through a crack cannot be patched with standard caulk or filler. UGL DryLok Fast Plug is a hydraulic cement-based product designed for exactly this situation. Mix it to a stiff consistency, press it firmly into the wet crack, and hold it in place for a few minutes as it begins to harden. DryLok Fast Plug sets quickly, so mix only small batches. Once the active leak is stopped, the wall can dry, be treated for efflorescence, and receive the standard waterproofing coating sequence.

Deteriorated mortar joints are repointed before painting. Rake out loose mortar to a depth of at least three-quarters of an inch, brush out dust, dampen the joint, and pack new mortar mix into the void in layers. Allow mortar repairs to cure for a minimum of seven days before applying any coating. Fresh mortar has a high pH that can cause adhesion failure in some coatings and will continue to lose moisture as it cures, which can cause blistering under a topcoat applied too early.

Efflorescence, the white crystalline deposit that forms on block surfaces from water carrying salts to the surface, must be removed before any primer or paint is applied. Efflorescence prevents bonding and will continue to grow under a coating, causing it to bubble and peel. Remove efflorescence mechanically with a stiff wire brush, then treat the affected area with DryLok Etch or a diluted muriatic acid solution. Muriatic acid requires protective equipment including chemical-resistant gloves, eye protection with side shields, and protective clothing. Always add acid to water, not the reverse. Rinse the treated surface thoroughly with clean water and allow it to dry completely before priming.

Block Filler vs Standard Primer for Masonry Surfaces

Block filler is not simply a thicker primer. It is a purpose-built product with a high solids content specifically formulated to fill the surface voids in concrete masonry units. A standard exterior primer, even a masonry-specific primer, does not have the body to fill block voids effectively. Applying a finish coat directly over block that has only received a standard primer produces a surface that looks textured and irregular, with the voids still visible as shadows through the paint film.

Sherwin-Williams PrepRite Block Filler is the professional-grade specification for concrete block walls. It is applied as the first coat in the three-coat system: one coat of block filler, followed by two coats of finish paint. PrepRite fills surface voids, creates a uniform surface profile, and provides the correct foundation for the topcoat to bond to. Apply it with a three-quarter inch nap roller, using heavy pressure to work the material into the surface voids. For rough block with deep surface texture, a one-inch nap roller may be needed to reach into the recesses.

Block filler requires a dry surface. Do not apply it to wet or damp block. After rain, allow at least 24 to 48 hours of drying time depending on temperature and humidity before applying block filler. The Portland cement content in the block absorbs and releases moisture slowly, so the surface may look dry before the subsurface moisture has fully cleared.

For standard masonry surfaces without severe voiding, BEHR Masonry Bonding Primer No. 880 is an effective alternative approach. This water-thin primer penetrates into the concrete surface, dries clear, and provides a bonding surface for topcoats. It is not a block filler and will not fill surface voids, but it is an excellent choice for denser masonry like poured concrete, brick, or stucco that does not have the deep void structure of block. Apply topcoat no sooner than four hours after BEHR 880 and no later than 30 days.

For new block construction, Sherwin-Williams Loxon XP Masonry Coating and Loxon Concrete and Masonry Primer Sealer are both formulated to handle high-alkalinity surfaces. New masonry can have a pH above 12, high enough to saponify many standard paint binders and cause adhesion failure. Loxon products are specifically engineered for alkalinity resistance. Apply to surfaces no sooner than seven days after block installation.

Spraying vs Rolling Paint on Rough Masonry Block

Rough masonry block has enough surface texture and enough void structure that rolling alone is slow and produces inconsistent coverage in the deep pockets of the surface. Spray application followed immediately by back-rolling is the professional method that achieves both speed and proper penetration.

Using an airless sprayer with a 515 or 517 tip, apply the topcoat at approximately 2,000 to 2,500 PSI in overlapping horizontal passes. Spray in sections, staying within a manageable distance behind the spray with a roller. Back-roll each section while the coating is still wet, using a three-quarter to one-inch nap roller and firm pressure to push the coating into the surface voids. Back-rolling after spraying accomplishes what rolling alone cannot: the sprayer delivers the material into the voids, and the roller consolidates it and ensures it is worked into the surface rather than bridging over the texture.

Without back-rolling after spraying, masonry block paint can bridge across voids without actually coating their interiors. This bridge-over pattern looks acceptable when first applied but fails early because the paint film is not bonded to anything inside the voids. Water and air intrude behind the film and cause bubbling and peeling long before a properly applied system would show any failure.

Pure roller application without spraying is viable but labor-intensive on block. Use a three-quarter to one-inch nap roller and work the material with significant pressure. Roll in overlapping passes, then cross-roll in the perpendicular direction to work paint into the full face of each block. Expect to apply more material per square foot to achieve the coverage that a spray-and-back-roll approach achieves in one pass.

Avoid applying masonry coatings in direct sun. Direct sun on rough block causes the surface to dry before the coating penetrates the voids, producing a surface coat that looks complete but lacks penetration into the texture. Apply in shade or choose morning hours when the block surface temperature is below 90 degrees Fahrenheit. In hot weather, dampen the block surface lightly with water before applying coating to slow the absorption rate and extend the open time for penetration.

Choosing Elastomeric Coatings for Exterior Block Walls

Standard exterior paint forms a rigid film. Masonry block walls experience micro-movement from thermal expansion, structural settling, and freeze-thaw cycling that can open hairline cracks in the block face faster than a rigid film can accommodate. Elastomeric coatings solve this problem by stretching with the substrate rather than cracking.

Sherwin-Williams Loxon XP Elastomeric is the commercial-grade specification for exterior block walls requiring crack-bridging capability and long-term waterproofing. Applied at five to ten times the film thickness of standard paint, elastomeric coatings stretch 150 to 400 percent as hairline cracks open and close with temperature cycling, maintaining a continuous watertight film. Loxon XP can be applied as early as seven days after block construction, handles surfaces with pH up to 13, and is available with a reflective pigment option that reduces surface temperature by reflecting infrared energy, relevant for dark-colored walls in high-sun climates.

The coverage rate for elastomeric coatings on rough block is significantly lower than standard paint. Where standard exterior paint covers 350 to 400 square feet per gallon on smooth surfaces, elastomeric on rough block may yield 80 to 100 square feet per gallon or less, depending on the application rate and surface texture. This is by design. The lower coverage rate delivers the film thickness required for the coating to perform its crack-bridging function.

One important limitation: do not apply elastomeric coatings to retaining walls that have active hydrostatic pressure from behind. Water pressure pushing from the soil side will work under the elastomeric film and detach it from the surface in sheets. For retaining walls, use a breathable masonry waterproofing system with proper drainage relief, not an elastomeric surface coating.

Loxon XP also carries a reflective pigment version that reduces surface temperature. On south- and west-facing block walls in hot climates, this can meaningfully reduce the thermal cycling the coating experiences, extending its service life.