Portland cement plaster is a versatile, durable facing material that can be applied to either exterior or interior surfaces of a structure. The serviceability of traditional portland cement plaster has been proof-tested through long use in extreme environments such as the cold temperatures of Alaska and Canada, the hot temperatures of the humid Gulf States, and the dry Southwestern States.
Portland cement plaster and portland cement stucco are the same material. The term "stucco" is widely used to describe material applied to exterior surfaces. Traditional portland cement stucco should not be confused with the Exterior Insulation Finish Systems (EIFS) which are sometimes called "synthetic stuccos."
Concrete masonry provides an excellent base for direct application of portland cement plaster. To enhance that inherent compatibility, select open-textured concrete masonry units that promote mechanical keying between the plaster and the base masonry wall. Cut all mortar joints flush, rather than tooling or sponge floating them. Because plaster and concrete masonry undergo similar volume changes, place control joints in the plaster only in locations that coincide with control joints in the concrete masonry.
Plaster is applied to concrete masonry surfaces in two or three coats, consisting of basecoat(s) and a finish coat. Prior to application of the basecoat, uniformly pre-moisten concrete masonry surfaces to aid in curing of the plaster. However, avoid saturating the masonry either by excessive spraying or by exposure of uncovered masonry walls or units to rain. If open textured masonry units are not available or the condition of the masonry surface is questionable, apply a dash bond coat of a non-re-emulsifiable bonding agent to masonry surfaces prior to application of the basecoat. A dash bond coat is a slurry mixture rich in cement, usually one part cement to no more than two parts sand, that is literally dashed against the wall using a brush, trowel, paddle, or machine.
With the right planning and preparation in the construction of the concrete masonry base, the final step is to follow the requirements of ASTM C 926, the Standard Specification for Application of Portland Cement Plaster, and the guidelines given in PCA's Portland Cement Plaster (Stucco) Manual EB049 in applying the plaster. The result will be a structure having the strength and durability of masonry with a popular, attractive stucco finish.