The repair of historic stucco should begin by identifying the cause of the damage to the stucco finish. Historically, the application of stucco was quite similar to the process of applying lime plaster. Repairs should be carried out as soon as problems become visible, as the damage will only become worse over time. Cracks may form in the stucco due to building settling or direct damage to the exterior coating. Once water is able to breach the coating, whether through an opening in the stucco itself or from beneath its surface, fragile stucco can begin to buckle and crumble. Wood is a common structural material that is often used as substrate beneath stucco. It can absorb moisture from at or below ground level and draw it away from the original source of the problem. Stucco can also be applied to masonry such as brick or stone, which can also be damaged by moisture infiltration.
Rising damp from groundwater or drainage issues is especially problematic. The stucco can delaminate from damp wood lath beneath and as the wood rots, the stucco may begin to deteriorate and separate from it and the building. Damage to the stucco itself leads to further moisture infiltration that exacerbates the deterioration of the finish as well as the substrate. Downspouts, gutters, flashing and other means of managing directing water away from the building will prevent damage from getting worse. Without proper guttering, water may splash up onto stuccoed surfaces, staining and accelerating the deterioration of the finish. Grading of the soil around the building may also be necessary to redirect moisture away from the structure and foundation.
Depending on the extent of the damage to the finish, varying degrees of repairs may be carried out. Small hairline cracks may be sealed with an additional layer of finish coat or even simply a coat of paint. Modern caulking materials are not ideal mediums of repair. Making the choice to patch or completely repair a stuccoed surface depends on the texture of the finish coat. Repairs, especially numerous ones, made to a smoothly finished surface will be more noticeable and recovering the entire surface with a new layer of finish coat may be more appropriate. Conversely, it may be easier to conceal patches of repair work to a textured surface and complete refinishing may not be necessary.
Preparation should begin with the removal of all damaged material in the area to be repaired. Any stucco that is loose should be removed as it has already failed. The removal of compromised materials may extend to wood lath or other substrates that may have also have been damaged, although it may be preferable to install new mesh over damaged lath. Care must be taken regarding this approach, as it may be especially critical when authenticity is of a concern regarding a historic building. In such cases, replacement of damaged lath is generally considered to be more appropriate than installing new mesh. All surfaces should be cleaned to remove paint, oil, or plant growth. Stone or brick mortar joints may be scored to a depth of 5/8" to allow for the proper adhesion of the new stucco. New stucco patches should not overlap old stucco.
To obtain a neat repair, the area to be patched should be squared-off with a butt joint, using a cold chisel, a hatchet, a diamond blade saw, or a masonry bit. Sometimes it may be preferable to leave the area to be patched in an irregular shape which may result in a less conspicuous patch. Proper preparation of the area to be patched requires very sharp tools, and extreme caution on the part of the plasterer not to break keys of surrounding good stucco by "over-sounding" when removing deteriorated stucco.
The application of new stucco should not include wire mesh when lime stucco is being repaired on a masonry surface. The new stucco repair should bond to the masonry substrate appropriately without mesh. The introduction of wire mesh has the potential to hasten deterioration of both the masonry and the stucco finish as the slightest amount of moisture will lead to rust developing on the wire mesh, which expands as it rusts. This may lead to the spalling of not only the new stucco, but also of the masonry itself.
After thoroughly dampening the masonry or wood lath, the first, scratch coat should be applied to the masonry substrate, or wood or metal lath, in a thickness that corresponds to the original if extant, or generally about 1⁄4″ to 3⁄8″. The scratch coat should be scratched or crosshatched with a comb to provide a key to hold the second coat. It usually takes 24–72 hours, and longer in cold weather, for each coat to cure before the next coat can be applied. The second coat should be about the same thickness as the first, and the total thickness of the first two coats should generally not exceed about 5⁄8″. This second or leveling coat should be roughened using a wood float with a nail protruding to provide a key for the final or finish coat. The finish coat, about 1⁄4″ thick, is applied after the previous coat has initially set. If this is not feasible, the base coat should be thoroughly dampened when the finish coat is applied later. The finish coat should be worked to match the texture of the original stucco.
paring for the repair process requires testing to determine the composition of the stucco that is to be repaired. Due to numerous factors, including regionally available materials and original workmanship, there is a variety of materials that may have been used for the original application of stucco. It must also be determined if the type of stucco used is lime-based or Portland cement-based. Of particular concern is the use of Portland cement, which is harder than previous methods used in stucco application. This material is not compatible with softer and more flexible lime cement that was used in the 18th and into the mid-19th centuries. Test sampling is critical to determine the best mixture, in terms of durability, compatibility, texture and color to use in the repair process. Test patches should be used to help make this determination. The same number of layers should be used in the repair as were used in the original stucco application.
Further research that should be conducted prior to the commencement of repair includes determining the types of ingredients used in the original stucco so that the color and texture of the original material can be matched as closely as possible. In some cases, shells or pebbles were used in the stucco covering. Regionally sourced sand, for example, may have been used in the original application, but may no longer be readily available. In this way, stucco tended to be tinted directly, although sometimes it was painted after being applied. Additionally, the use of ingredients such as animal hair was popular in some regions. Care should be taken that repairs made include similar ingredients that are clean and free of oils. This is another reason for the use of test patches, to ensure that the repairs blend into the original building fabric as much as possible.