As a former plasterer, contractor and EIFS sales manager, I have come to learn that exterior wall textures, like color, are viewed differently by each person. Exterior wall textures can range from very smooth to heavily textured. It’s when one chooses the “smooth” texture that the views become more critical and varied.
In the earlier years of plastering exteriors, the smooth look was done on most plastered buildings. Think of the Adobe plastered walls on an old church. These smooth finishes were achieved in a variety of ways such as using lambs wool, wood trowels or cloth and typically had sandy and smooth areas as textures in the same wall area. Because they were built of earthen materials and plastered by hand, they were wavy and not very true in plane like our modern buildings of today. As buildings became more modern and larger in wall area, the skip-trowel, river sand or heavily textured walls became the texture of choice. Because of these larger textures, hairline cracks, color variations and surface undulations went largely unnoticed. But also because of the heavy textures, dirt build up on the textured surface could make the walls appear stained or dull and dirty. Thus began the rebirth of the smooth finish. It is in these smooth textures that the modern perception differs from that of the old perception.
Today, in our modern exterior smooth wall finishes, there are two finish products commonly used: cement finish and acrylic finish. Cement finish can only be applied to stucco cement basecoats and concrete wall assemblies. These finishes are applied to a thickness of approximately 1/8 inch. Although cement finishes are available with integral coloring, these colored finishes should not be used in smooth slick troweling applications, as this process tends to leave color mottling, unless that is the intent of the designer. To achieve a desired finish color, the smooth finished stucco surface could be painted.
Acrylic finishes can be applied to stucco cement, concrete assemblies and acrylic basecoats typically used in EIFS systems. Most acrylic finishes are integrally colored; however, some brands are made to be painted to achieve the desired color. With acrylic finishes the thickness is determined by the size of the largest aggregate. Because the smooth acrylic finishes have very small aggregate sizes, two coat applications are typically required.
CEMENTSmooth cement finishes require a higher level of trowel experience and are the most time consuming type of cement finish. They are typically applied in multiple layers with the first coat being applied as a “scratch” layer. The scratch layer fills most voids in the stucco basecoat and begins the leveling process to achieve the smooth look. It also cools the surface to allow more working time on the second coat and finish slicking process. A key element to remember in smooth coat stucco is that the scratch layer of finish should not be wet troweled or slicked out as this may create a blistering issue in the second coat. The second pass is applied soon after the first or scratch layer has taken up to sufficient hardness so as not to disturb it during the second pass application.
Timing is everything for wetting and slicking out the second (or third if necessary) coat. Misting with a water bottle moistens the cement surface sufficiently to begin slicking out the surface. Trowel too soon and the water just re-wets the stucco leaving trowel marks, pits and chatters; too late and you end up troweling over a hard surface with no slicking effect. The key to a truly slicked-out cement finish is in getting the “fat” to the surface to fill in the pits, marks and holes left from spreading. (“Fat” is what is left on the trowel surface after wet troweling the aggregate deep into the finish layer.) The fat is then used for filling and smoothing. It is the fat filling the surface voids and marks that creates the slick cement finish look. The final appearance should feel very smooth to the touch and exhibit a slightly undulating, mark free surface.
ACRYLICSmooth acrylic finishes also require a higher level of trowel experience. An acrylic finish cannot be truly slicked out like a cement finish can. Because of the nature of the acrylic and aggregate mix, very little or no fat is created during the smooth troweling phase. Like cement, this application is done in two passes with the first being a scratch coat and the second being the smooth finish surface.
Unlike cement, acrylic finishes should be applied in two thin layers and not be applied in three or more coats. Excessive coats in acrylics can cause blistering (always check with the acrylic manufacturer for a best practices guide). Even more critical then is the basecoat application as it relates to flat and level. A minimal amount of water should be spritzed onto the surface for troweling. An excessive amount of water can compromise the integrity of the acrylic finish. Again, timing is critical in the final troweling of the second coat. Too much water and the finish becomes diluted and quite possibly damaged, too dry a surface and the acrylics will burn leaving dark trowel marks on the surface.
It is because of the minimal amounts of water able to be used and lack of fat that acrylic finishes cannot be truly slicked out. Smooth yes, but with a slight texture that can be felt when rubbing your hand over the surface. Smooth acrylic finishes can also be floated with a hard plastic float to create a smooth look. Floating leaves slight marks and texture variations than does troweling, but is easier to do and has a smaller margin of error.
Many modern design professionals, homeowners and building owners desire a smooth or slick finish. Their expectations of the final smoothness may be higher than the smooth finishes are able to provide. Consideration should be given to provide actual material samples on the walls to be finished, or on mock-up walls, to establish a final acceptable appearance.
As with all smooth surfaces on a building, even glass, a phenomenon known as “The Witching Hour” may make the smooth surface appear not so smooth. The witching hour is where the sun, at a given hour, washes light directly down the wall showing every undulation, mark, or trowel stroke left in the slicking process. It is this phenomenon that creates the heartbreaks and heartaches in the plastering industry. The appearance does not necessarily mean that the applicator has done substandard work or that the surface is not smooth. It just makes the wall surface appear to be less flat and smooth. Another consideration for smooth wall finishes is that because they are smooth, small cracks, tiny bumps and the slight undulating surface left by the hand motion during slicking, all become more visible in the final product, compared to a textured wall.
REMEMBER THISOne thing to always keep in mind is that once a finish is smooth troweled, any damage and subsequent patching would probably show through (remember the Witching Hour). If any patching is required on a given wall plane, usually the entire elevation from corner to corner will need to be re-done. It is very difficult to patch a smooth wall surface without leaving some noticeable ridge or color variation. Most applicators will not even try to patch the surface and opt to re-spread the entire wall. In smooth or slicked stucco walls, the surface will need to be roughed up a bit to ensure good adhesion. In acrylic finished walls, care should be taken not to get the finish surface too thick. I recommend consulting the acrylic finish manufacturer for their specifications. All of these patching considerations are important when evaluating whether or not a crack, for instance, is going to be patched or not. Sometimes it is easier to leave the crack alone than it is to patch and re-spread the entire wall corner to corner.
Smooth finishes provide great architectural appeal and give that clean look to a building. Understanding the degree of smoothness that the exterior finishes can provide and what they ultimately look like is important. Remember that the smoother the surface, the more visual the imperfections. We are asking our smooth finish product to look good and perform as a durable exterior cladding protecting our interior space. Setting our expectations accordingly will provide years of worry free, architecturally pleasing, smooth wall surfaces.