Technical Memo:  Smooth Trowel Stucco Finishes

Most stucco manufacturers produce a product that can be trowel applied to create a smooth or semi-smooth finish for use over portland cement plaster “brown coat.” These smooth finished products are a blend of cement, hydrated lime, aggregates and proprietary ingredients. They are designed to be trowel applied to create a smooth uniform finish that is regular and even in color.

Smooth Trowel Stucco can also be used to produce a finish with variegated or mottled color. This popular look, often called Mediterranean or Tuscan, is a result of special finishing techniques employed by the applicator. Variegation or mottling in a colored portland cement finish occurs when the applicator trowels the finish repeatedly and adds water to the surface of the finish during the process. This water toweling technique produces an old world patina that is increasingly popular.

This technique brings much of the binder (sometimes called “paste” or “fat”) in the product to the surface, separating it from the aggregate. This repeated toweling also builds up heat from the friction and burns or burnishes the finish, leaving light and dark areas on the surface.

Check cracking, sometimes referred to as eggshell- or spider-cracking, is also a common result of this technique. This cracking does not impact the performance of the finish and should be considered a normal element of the desired aesthetic.

It should be noted that water toweling also leaves the surface of the finish very dense; this prevents the stucco from being refinished without extensive surface preparation because of the lack of a mechanical key.

The raw materials that are used in the composition of Smooth Finishes sometimes contain small pieces of lime, pigment or cement that are not detectable during manufacturing but that can contribute to color streaks or spots when the material is over toweled. This cannot be controlled during the manufacturing process. The applicator has the option during the application process to remove material that may be undesirable or to repair the finish after installation if the property owner so requests.

It could be determined that a color issue is the result of a manufacturing error. In such instances, the stucco manufacturer can only be responsible for correcting the surface to the original, uniform color. Manufacturers cannot assume the responsibility of duplicating or matching finish to resemble a variegated or mottled look.

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