Please note the difference between the blocks in this unpainted masonry fence wall. Concrete masonry units (CMUs) vary greatly in profile depth and porosity. Most people, including the average painter, won’t notice the porosity differences. But once the wall is painted, most homeowners will complain that there are holidays (misses) in our workmanship. But that would be a misdiagnosis of the problem. What is the problem? It is the varying porosity. Different concrete blocks soak up the paint differently. If the blocks were exactly the same, the paint would have covered each block in a near identical fashion, and the wall would have a uniform appearance.
When light reflects off the surface of a painted wall unevenly, that is called flashing. Paint flashing is an uneven appearance of a paint coating’s gloss, sheen, or luster. It can happen for various reasons, such as using the wrong type of paint, exposing the paint to moisture or temperature changes during drying, applying the paint at uneven thickness, or, as in this case, painting over a porous surface. To fix paint flashing, you may need to apply a second coat of paint, prime or seal the surface, use a better-quality paint or roller, or thin the touch-up paint to blend it better.
MTS Painting can recommend how to handle the issue of an inconsistent appearance in painted masonry block walls. The first step is to prepare the porous service to receive an even topcoat by priming the wall with a block filling product. According to Painting Contractors Association Industry Standards (PCA P3.1.27) Block Filler is a thick, medium to high solids heavily pigmented material used for application on concrete blocks for filling and smoothing the surface for subsequent finish coatings. The industry standard defines three levels of block filling. Level 1, Economy Fill, is recommended in spaces not occupied by the public, such as stairways in buildings. Level 2, Standard Fill, is recommended in spaces that are occupied. Level 3, Premium Fill, is recommended for walls that have to be cleaned or disinfected to satisfy health regulations, like public restrooms.
Block filling is considered an upgrade. Most new construction concrete block walls are primed with a standard concrete primer. We use Seal Krete. However, when a smooth wall is desired or required, we use a product by Sherwin Williams called PrepRite.
Block filler primer can be brushed on with a nylon-polyester brush, rolled on with a deep nap roller cover, or sprayed on with 2300 psi pressure and a .028-inch spray tip. It applies much thicker than typical acrylic wall paint at 16 mils wet, which dries to about 8 mils thickness. When spayed on, the typical process follows the spraying immediately with back rolling. Back rolling is a method where freshly applied paint (wet) is smoothed out with an undipped roller to even the appearance, fill small voids, and improve uniformity. This process will produce a relatively smooth surface. Two coats properly applied may be required to provide the most pinhole free, uniform surface. Squeegee or trowel will provide the Level 3 Premium Fill finish.
The following are excerpts from the PCA Industry Standards concerning block filler.
PCA P12 – Levels of Block Filler
- 1.1 This Standard establishes consistent procedures for the specification of block filling and the application of block filler prior to painting paint grade smooth face concrete masonry units.
- 1.2 The purpose of this Standard is to prevent misunderstandings and conflicts when the application of block filler is required.
5. Standard Specification
- 5.1 The degree of block filler achieved should not be assessed until all specified paint coats have been applied, as the finishing coat(s) will contribute to the degree of fill. It is recommended that a Benchmark Sample showing specified level of block fill on entire sample and finish coat(s) on a portion of the sample be prepared and approved according to PCA Standard P5 to demonstrate the result of block filling to the specified level.
- 5.2 The use of some materials and some block profiles cause air entrapment resulting in unavoidable pin holes. A void or discontinuity visible to the substrate is a system defect.
- 5.3 Levels of Block Filler
- 5.3.1 Level 1 – Economy Fill: One coat applied with equipment specified by the coating manufacturer. This level reduces the quantity of paint required for succeeding paint coats. It reduces some irregularities in masonry profile depth. It is normal that voids will remain, depending on the porosity and profile depth of the block. The block filler shall be applied at the spreading rate recommended by the manufacturer. This level is normally used in spaces that are not occupied by the public and in stairways of high-rise buildings.
- 5.3.2 Level 2 – Standard Fill: One coat applied with equipment specified by the coating manufacturer. Back rolling will be performed as necessary to attempt to fill deep irregularities. Masonry profile depth will be slightly reduced. Joints will be visible as tooled. The number of voids will be minimized, but voids may remain depending on the porosity of the block. A maximum of ten voids per square foot of surface area shall be deemed to be acceptable. The block filler shall be applied at the spreading rate recommended by the manufacturer. This level is normally used in finished areas that are occupied by the public.
- 5.3.3 Level 3 – Premium Fill: Two or more coats of high-performance block filler manufactured to be applied at a high dry film build. Block filler shall be back rolled to eliminate voids and reduce the majority of the masonry profile depth. This system, with an appropriate paint finishing system, produces a surface that is easier to clean to meet health regulations. Exterior use of this level of block filler, with an appropriate paint finishing system, will reduce water intrusion at exterior walls.
- 5.4 Specifications not specifically stating the level of block filler to be attained will be assumed to imply a Level 2 Standard Fill.
- 5.5 The acceptability of the Level 2 Standard Fill surfaces shall be determined when viewed without magnification, at a distance of thirty-nine (39) inches or one (1) meter or more, under finished lighting conditions and from a normal viewing position.
- 6.1 Since the number of coats and application techniques required for block filling impact project costs, the painting specification should incorporate the appropriate level(s) of block filler as defined by this Standard.
- 6.2 The masonry specification should properly define all cleaning and detailing of new masonry prior to block filler and painting work. Unless specified otherwise, this work is not the responsibility of the Painting Contractor.
- 6.3 The result of any particular level of block filler will vary due to the fact that concrete masonry units vary greatly in profile depth and porosity. The filling of holes is not the responsibility of the Painting Contractor and should be pointed up by others prior to the application of block filler.
- 6.4 Proper specification of masonry materials, such as requiring graded aggregates to minimize masonry porosity, is required to achieve the best appearance and performance of the block filler and paint system specified. This Standard outlines procedures for on-site determination and approval of achievable quality from specified paint and coating systems applied to paint grade smooth face concrete masonry units.
- 6.5 This Standard clarifies areas of responsibility. Improved communication reduces misunderstandings.
- 6.6 This Standard is a nationally recognized consensus document for the painting and coating industry’s work practices.