Applied like plaster, stucco is a mixture of sand, portland cement, lime, and water.
Two coats are applied to masonry, three coats on metal lath. The finish coat may be tinted by adding coloring matter to the mix or the outside surface may be painted with a suitable material.
The metal lath should be heavily galvanized. It should weigh at least 2.5 lb / yd2, even though furring strips are closely spaced. When supports are 16 in c to c, it should weigh 3.4 lb /yd2. (See Table 11.9 in Art. 11.25.6). The lath sheets should be applied with long dimensions horizontal and should be tied with 16-ga wire.
Edges should be lapped at least 1 in, ends 2 in.
The first, or scratch, coat should be forced through the interstices in the lath so as to embed the metal completely. In three-coat applications, the coat should be at least 1⁄2 in thick. Its surface should be scored to aid bond with the second, or brown, coat. That coat should be applied as soon as the scratch coat has gained sufficient strength to carry the weight of both coats, usually after about 4 or 5 hr from completion of the scratch coat. The second coat should be at least 3⁄8 in thick. It should be moist cured for at least 48 hr with fine sprays of water and then allowed to dry for at least 1 week. The finish coat should be at least 3⁄8 in thick. (When only two coats are used, for example, on a masonry base, the base coat should be a minimum of 3⁄8 in thick and the finish coat, 1⁄4 in. Before application of the base coat, a bond coat, consisting of one part portland cement and one to two parts sand, should be dashed on the masonry with a stiff brush and allowed to set.)
For both the scratch and brown coats, the mix, by volume, may be 1 part portland cement to 3 to 5 parts sand, plus hydrated lime in amount equal to 25% of the volume of cement. Masonry cement may be used instead of portland cement, but without addition of lime, inasmuch as masonry cement contains lime. The finish coat may be a factory-prepared stucco-finish mix or a job mix of 1 part white portland cement, not more than 1⁄4 part of hydrated lime, 2 to 3 parts of a lightcolored sand, and mineral oxide pigment, if desired.
Ingredients should be thoroughly mixed dry. Then, water should be added and the materials mixed for at least 5 mm in a power mixer. The first two coats usually are applied with a trowel. The finish coat may be sprayed or manually applied.
(‘‘Plasterer’s Manual,’’ EBO49M, Portland Cement Association.)