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Proper Mixing Is Crucial

The mixing of stucco must be carefully controlled (Figure 3).

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Figure 3. Close attention must be paid to mixing stucco to ensure a uniform and durable coating.

In a perfect world, "proportioning boxes," which ensure a perfect ratio of ingredients, would be used whenever a batch of stucco was mixed. This would guarantee consistent, uniform batches of scratch, brown, and — most important — finish mud on every layer of every job. Regrettably, this is rarely the case. The guy on the mixer can get interrupted by a question, a cigarette, or pausing to drink from the cooler. As a result, he loses track of how many shovels of sand or cement or gallons of water he’s put in the mixer, and the outcome is a mismatched batch. This can reveal itself as a different color or texture, and even years later, it can effloresce or worse. Other consistency rules include sizing batches for complete use within one hour after mixing, and withholding 10% of the mixing water until mixing is almost complete (strive to keep water to a minimum). Pigments, especially in the finish coat, must be mixed carefully and consistently. The mixer must be allowed to run until the color is dispersed throughout the entire load of finish material. Undermixing the finish will permit lumps of raw color to be left in the finish, and the results will be horrific.

Substrate Concerns

Even if you have a robot at the mixer producing consistent, uniform batches of finish material every time, the color coating will be affected by the absorption (relative water-drawing potential) of what it’s being applied to. If you apply finish mud over a 10-day-old brown coat and an adjacent surface of 15-year-old concrete block, for example, the different rates of absorption will cause a radical shift in color where the two adjacent surfaces meet — in spite of the batches being uniform. Unlike its thicker cousin, concrete, stucco is not tolerant to movement of the underlying surface. Build a wall of truly inert material, such as concrete block, and you can guarantee no cracks. Build a house out of today’s third-growth, guaranteed-to-warp-and-split lumber and there’s no question: The stucco covering will crack. The point is, in wooden structures, you can safely predict vertical hairline cracks in stucco that are in no way the fault of the stucco contractor. Nevertheless, everything possible should be done to avoid lathing over wet wood framing and sheathing. Even water from the stucco mix can wet the wood enough to cause it to expand. When applying stucco over wood framing, always use a layer of a water-resistant paper such as grade-D paper or housewrap. The Uniform Building Code, enforced in most western states, requires two layers of grade-D paper over wood sheathing.

No Lathing Matter

It’s the paper and wire guys who set the tone for the correctness and profitability of the whole stucco job (Figure 4).

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Figure 4. Careful lath and flashing installation sets the tone for a high-quality job.

They know they’ve got to get the job done because there’s a mud crew breathing down their necks. So it falls to them to read the prints and determine where the stucco is to be applied, cope with imperfections in the work of prior trades (such as block layers and framers), and flash and mount stucco trim pieces so the job is both durable and follows the prints.