Changing Faces

April 1, 1998
Concrete has long been an excellent building material. Strong and durable, it offers long-term value and often provides a suitable exterior appearance.

Concrete has long been an excellent building material. Strong and durable, it offers long-term value and often provides a suitable exterior appearance. Despite its rock-hard appearance, concrete is subject to discoloration. A fresh coat of paint can be used when color uniformity or color enha ncement of a concrete wall is important for aesthetic reasons. But, that is not the only benefit paint offers. Paint can help prevent the emergence of efflorescence-a white, crusty substance that can mar the appearance.

Preparing the surface Surface preparation is essential to a successful, long-lasting job. The surface needs to be sound; free of dirt, mildew, oil, powdery dust and loose chalk; and rough enough to ensure good adhesion of the coating.

Weathering is the most desirable means of preparing concrete for painting. This natural process allows the concrete to dry out, reduces the surface alkalinity, and eventually adds a beneficial roughening to the surface. However, this aging process can take up to a year, and often it is impractical to delay painting for that long.

If you need to paint concrete that is less than a year old, use 100-percent acrylic latex paint. Only four weeks of aging are required with these paints because of their resistance to surface alkalinity. However, an alkali-resistant masonry sealer or primer is still suggested if the surface is less than a year old, and highly recommended if it is less than four weeks old. In the case of oil-based or alkyd paints, an alkali-resistant primer or sealer is always necessary before painting if the concrete has aged less than a year.

Things to remember There are a number of things you need to be familiar with when painting concrete, especially new or insufficiently aged concrete: *Allow excess moisture to escape. Moisture that remains in concrete from the original mixing needs to migrate to the surface and evaporate. To ensure that this moisture, as well as any interior humidity in the structure, can escape, use a latex paint because it allows for vapor transmission. Paint that is impermeable to water vapor, such as an oil-based or alkyd paint, tends to fail by blistering and peeling because the moisture trapped in the concrete applies pressure to the paint from behind. This internal pressure can lift paint from the surface of the concrete in bubbles or blisters. This property, in combination with its limited alkali resistance, makes oil or alkyd paint unsuitable for use directly on fresh concrete.

*Choose alkali-resistant paint. Concrete, like stucco, is generally quite alkaline, especially if it's less than a year old. This highly alkaline surface can cause premature fading of alkali-sensitive paints. It can also promote efflorescence. When dealing with new concrete, choose paints that are able to resist the alkalinity of the surface. Top-quality latex paints made with 100-percent acrylic binders generally are resistant to alkali, have good hiding characteristics and are durable, again making them a good choice for concrete walls. They also are easy to apply by brush, roller or spray, and dry quickly.

Cementitious patching compounds sometimes are applied a few days prior to painting to smooth imperfections in the concrete. Because they are fresh, the patches are usually more alkaline than the wall, and should be primed with an alkali-resistant sealer to preclude failures.

*Remove efflorescence. If there is any efflorescence present on the surface, it must be removed prior to sealing or painting. Otherwise, these salty deposits may interfere with paint adhesion. These deposits also may work their way through the paint film and ruin the appearance of the finished job.

Efflorescence often can be removed by thoroughly scraping and wire brushing the surface. More stubborn efflorescence can be removed by sandblasting the surface or etching it with a 6- to 10-percent solution of muriatic acid. If using the latter process, wet the surface with water before applying the acid solution. Allow the acid to remain on the surface for about five minutes, and then scrub the surface with a stiff brush before rinsing with water again. Whenever using muriatic acid, protect eyes and hands, and rinse the surface thoroughly after cleaning.

*Roughen smooth surfaces. Inspect the entire area to be painted to make sure there are no highly glazed surfaces. Smooth, tight surfaces may not provide sufficient voids for adequate paint penetration and adhesion. They also tend to accentuate lapping, especially at certain sun angles.

If smooth surfaces are present, they may have to be roughened to ensure good adhesion. In most cases, light sandblasting will do the job.

*Remove oily contaminants. Forms used for casting concrete often are coated with a release agent. To ensure that the surface is completely free of form-release agents, remove any oily contaminants by either power washing, light sandblasting or etching the surface with an acid solution and a small amount of detergent. Always thoroughly rinse the surface with clean water before painting to wash away all loose debris.

Previously painted concrete When painting previously painted concrete, be sure to remove all dirt, mildew, loose paint, powdery dust and other loose material from the surface. Power washing is an effective approach. Bristle brushing followed by hosing with clean water also is effective. If necessary, brush or lightly sandblast the area.

If you find mildew on the surface, a mixture of three parts common household chlorine bleach to one part water can be used to remove this contaminant. Again, be sure to rinse the area thoroughly with water. To help inhibit the growth of new mildew, use a high-quality acrylic latex paint. This type of paint generally contains extra mildewcide and resists mildew better than oil-based paints or lower-quality latex paints.

One reason brick is chosen as an exterior wall material is that its color is an integral part of the material itself. However, with proper surface preparation and quality paint, brick can be given a fresh, new appearance.

When preparing the surface of old, unpainted brick, the first thing to do is look for signs of efflorescence and remove it. If there are any cracks, these must be repaired. Old and weathered brick is extremely porous and will absorb paint quickly. To avoid this, prime the surface with a masonry sealer.

If moss has accumulated on damp, shaded brick, apply weed killer. However, wet the wall with clean water before applying to help prevent it from being drawn into the wall. Chemicals in weed killers may contain solubles that can contribute to efflorescence or react unfavorably with paint, and should be removed after application by scrubbing with a stiff brush.

If the the brick has been painted previously, surface preparation may require a few more steps. If mildew is present, remove it with a mixture of one part household-strength bleach and three parts water, and rinse thoroughly. Mortar that is less than a year old can be alkaline and can burn or deteriorate the binder of an ordinary paint, causing the color to fade and the paint to crack and peel. However, a top-quality paint made with a 100 percent acrylic binder will resist this initial alkalinity, allowing new mortar to be painted after only 30 days. If painting must be done before 30 days, use of an alkali-resistant masonry primer is highly recommended.

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