The finish coats of stucco may be divided into three general groups according to the texture and method of application. These are, first, dash finishes of the wet and dry type; second, the smooth finishes comprising the various modifications of the float finish; and, third, the exposed aggregate or surface treated concrete.
The wet-dash finishes include the "rough cast" or the "pebble dash," which is obtained by throwing with a paddle a mixture of cement grout and pebbles of a definite size against a fresh coat of mortar. The "spatter" dash is obtained in very much the same manner as the rough cast except that a very thin mixture of cement and coarse sand or stone screenings is dashed against the fresh mortar. The "sand spray" or "broom dash" is obtained by applying a creamy mixture of cement and sand with a whisk broom or a long fiber brush. The broom is dipped into the grout and then struck across the forearm or a stick held in the left hand, spraying the mixture on the finish coat of mortar.
1 For additional information on concrete see Design and Control of Concrete Mixtures (Portland Cement Assoc).
2 From Stucco Investigations at the U.S. Bureau of Standards with Recommendations for Portland Cement Stucco Construction (Circular of the Bureau of Standards, No. 311, 1926), pp. 27-32. See this publication also for uses of stucco for construction purposes, mixing and application, maintenance, and other information.
All these finishes are of comparatively low cost and are readily executed by workmen of ordinary skill. Owing to their rough texture they have the advantage of hiding the fine shrinkage cracks which develop to a greater or less extent in Portland cement stucco on hardening. These features contribute to the widespread use of the wet-dash finishes, and for the usual run of stucco work they are recommended.
There is considerable objection to the wet dashes because of their dull and cold uniform cement color. This objection may be met by using the white cement in the finish coat and dash or by tinting with mortar colors.
The dry-dash finishes are generally obtained by throwing with considerable force clean pebbles, stone chips, or pieces of shell against the finish coat before it has hardened. The aggregate should be largely of one size and should be uniformly distributed over the surface. The pieces may be pushed into place by the use of the float, but there should be no rubbing of the surface after the pieces are embedded. This finish is quite difficult to execute properly, but when well done it produces an acceptable color and texture quite different in character from those of the wet dashes.
The sand-float finish is produced by carefully floating the finish coat after it has taken its initial hardening. A lean finish coat is necessary, and it should be carefully straightened before floating is started. After the stucco has well stiffened, water is dashed on it by means of a brush and the final floating carried out until the sand tones predominate. It is probably the most difficult of all the finishes to execute and obtain acceptable surfaces. As the surface is smooth, imperfections of workmanship and defects show very conspicuously, and this finish should only be undertaken by workmen with considerable skill and experience.