As was discussed in How to Stucco | Mixing Stucco, applying exterior stucco is not rocket science. Knowing the process of mixing stucco correctly makes applying exterior stucco much easier. Many contractors put their newest employees on the mixer. Your stucco application crew will do a better and more efficient job if you put one of your best employees on the mixer. One of the major secrets to applying stucco is mixing stucco so every batch has the same perfect consistency. If you can do this, the crew with the trowels in their hands will have less to learn about how to apply stucco. This article does not address lathing, nor does it address waterproofing at transitions between
Note: This stucco application video shows the contractor pulsing the air valve. You can hear the “tsh, tsh, tsh” in the video. This allows him to achieve a 1/8th inch thick stucco finish coating. This process is also a good approach for spraying surface bonded cements.
Stucco can be applied with a hawk and trowel, with a pneumatic system, or it can be thrown at the wall with a shovel. After application, the stucco needs to be densified with a trowel and then smoothed out with long, even trowel strokes. The concept is to place the stucco in close contact with the substrate so there are no voids between the stucco and the substrate and so the stucco does not move after it is placed. If 30 minutes after the stucco application the lath telegraphs through the stucco, the stucco is sagging. This is caused by too much water, or by applying stucco too thick. Correct the problem before you proceed. The goal is to minimize the water in the mix while still maintaining a workable mix.
If you are applying stucco with a hawk and trowel, start with about one quart of stucco on the hawk and practice picking it up with the trowel and applying stucco back on the hawk. After you are comfortable with the process, place the lower edge of trowel against the wall and with the upper edge of the trowel extending out about 2 inches. Move the trowel up the wall while applying pressure to the wall. When you are close to running out of stucco, change directions slightly and lift the trowel as you run out of stucco. This breaks the suction and leaves the stucco on the wall. You now know how to apply stucco. All of the rest is refinement of technique. Pick up another quart of stucco and do it again. Keep practicing. Don’t drop the stucco. After you are comfortable, start loading your hawk with two quarts of stucco. Before long you will be building muscles and skills so you can handle about a gallon at a time. Always start your new trowel full at the bottom of the panel or over existing stucco. In that manner you do not leave voids behind the lath. For traditional three-coat work, try to apply the first coat at 3/8″ thick. Now show someone else the process of applying exterior stucco. As we teach, we learn.
With a stucco sprayer, applying stucco is much easier and faster because the stucco is propelled towards the substrate with air. There will be some bounce-back, but most of it sticks if it is mixed right. After the man with the sprayer moves on, a crew with trowels smooths the stucco out and ensures it is keyed into the lath.
After the first coat is on, a straight rod (Darby or browning rod) should be used to ensure that the stucco does not extend out beyond the lips of the casing bead and weep screed. The first coat is traditionally scratched horizontally to assist the second coat to bond to the first coat, thus the first coat is often called the scratch coat. Tests I have run, but have not published, have shown that back-dragging with the trowel to open up the first coat so it can breathe allows as much bond development as if the first coat were scratched.
The first coat needs to be moist-cured for 48 hours. This is light misting of the stucco to limit evaporation. If it is done right, it will remain a dark gray-green color for the cure period. Some standards allow the second coat to be applied as soon as the first coat can support it without cracking. As a result, some people apply the second coat within 3 or 4 hours of applying the first coat. I have found that if you delay the second coat until the first coat has cured for 48 hours, the amount of cracking of the second coat is substantially reduced.
The second stucco coat is applied in the same manner as the first coat. Care should be taken so the second coat does not extend out much beyond the lips of the casing bead and the weep screed. After the second coat is applied, a straight rod is used to shave any excess stucco off and leave a wall that can receive a finish coat of stucco. Do not polish the stucco surface it reduces bonding. Back-dragging with the trowel provides a better surface for the finish coat to be applied. After the second coat has been moist-cured for 48 hours and ambient-cured for 5 to 26 more days, depending on the local codes and the finish coat selected, the finish coat can be applied. In the western U. S. a traditional colored cementitious coating is very popular. In much of the rest of the country an acrylic or elastomeric finish coat is more common.
The cementitious finish coat can be applied after the second coat has cured for 7 days. Most of the acrylic and elastomeric coating manufacturers specify that one must wait for 28 days before applying the finish, must test the wall to insure the pH is below 9.0 or 9.5, or must treat the wall with a neutralizing sealer. The reason for the delay is twofold: it gives the calcium ions a chance to surface and combine with carbon dioxide from the air, and it allows the pH of the wall to drop. High pH interferes with the bonding of some elastomerics and acrylics.
Do not try to apply a perfectly-smooth finish coat. That is the hardest kind to do. Stucco textures were developed to cover up imperfections of wall services. If you use a cementitious finish coat, one good option is applying a stucco fog coat after about 2 to 3 weeks. A fog coat is the finish coat without any sand. It is often sprayed through a Hudson Sprayer to remove the mottling caused by efflorescence on the wall.
Another secret of how to apply stucco is to remember that it is easier to prevent efflorescence by taking your time and letting the wall cure than it is to come back and scrub the wall with an acid wash to remove the efflorescence.
Cracks in stucco do not just happen. They are caused. As you become more adept at applying exterior stucco, you will learn that foundation problems, failing to install control joints correctly, loose lath, failing to gap wood substrate, nailing sheet rock onto the interior side of the wall after the stucco is in place, using too little sand or the wrong gradation of sand, using the wrong amount of water, and applying a second coat before the first coat is cured enough to
support the trowel pressure, can cause cracking. Repairing these cracks is the subject for another guide. It is better to take the time and learn how to apply stucco without any cracks developing.
A well-done stucco job will last for many decades. Each year, look it over, wash it off, and inspect the interface between the stucco and other materials (like windows) to ensure that sealants remain in place.
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