TECHNICAL INEORMATION ON BUILDING MATERIALS

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FOR USE IN THE DESIGN OF LOW-COST HOUSING

THE NATIONAL BUREAU OF STANDARDS

UNITED STATES DEPARTffiNT OF COMi/ffiRCE

WASHINGTON, D. C.

July 15, 1936.

FINISHES AND MAINTENANCE

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PORTLAI'U) CEMENT STUCCO CONSTRUCTION

This is a digest of apart of Circular No, 3H. "Stucco Investigations at the Bureau of Standards with Recommendations for Portland

Cement Stucco Co'^struction" , (December 13, 1926),^ by Franlo A. Hitchcock, dealing witn finishes and maintenance of portland cement stucco

construction.

FINISHES FOR STUCCO

Finish coats for stucco may be divided into general groups

according to the texture and method of application as follows:

Wet Dashes ; Included are the "rough cast" or "pebble dash"

obtained by throwing with a paddle a mixture of cement grout and pebbles of a definite size against the fresh coat of mortar. The

"spatter" dash is obtained in much the same manner 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 whi^o broom or a long fiber brush dipped into the grout and then struck across the

forearm or a stick held in the left hand, which sprays the mixture on the finish coat. These wet dashes are conroaratively low in cost and

readily applied by workmen of ordinary skill. Their rough texture has

the advantage of hiding fine shrinkage cracks which may develop in

Portland cement stucco on hardening. For the usual run of stucco work.

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Available from Superintendent of Documents, Government Printing

Office, Washington, D. C. (Price 15 cents)

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the foregoing .treatments are recommended. Objections to the wet dashes

because of their dull and^ cold uniform cement color may be overcome by

using white cemeht in the finish coat and dash or by tinting them with

mortar colors. ,,

Dry Dash ; 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 hardens. The aggregate should be

mostly of one size, be uniformly distributed oyer the surface and should

be pushed into' place by the use’ of a float to embed the pieces in the

surface. This finish is. quite difficult to apply properly, but when

well done produces a color ' and texture different in character from those

of wet dashes.

Float Finishes ; Sand-float finishes are produced by carefully

floating the finish coat after it has taken its initial hardening. A

lean finish coat is necessary and should be carefully straightened before floating is started. When the stucco has well stiffened, water

should be dashed on by means of a brush and the final floating carried

out until the sand tones predominate. This smooth surface is probably

one of the most difficult to obtain and since imperfections show rather

conspicuously, it should only be undertaken. by skilled workmen.

Textures ; Unlimited possibilities in finishes and textures,

avoiding the monotony of the dashes and the difficulties in executing

float finishes, may be had by the introduction of colors or combinations

of colors. A finish lending itself to a two color combination, better

known as the ^'floated rough-cast" may be obtained by preparing the wall

the same as for a rough-cast finish and then li^tiy smoothing off the

hi^ points with a wood float. A finish coat of one color is next

applied and partially floated. While still plastic, another color coat

is then dashed on and the hi^ points again smoothed off blending the

two colors together.

Another finish, the " sponge- float " , is produced by carefully

working the plastic finish with a soft wood float drawing the float ' away from the surface at random. The suctiqn produced gives a roughness of surface, the texture of which is between that of the wet dashes

and the smooth' sand- float finish. ’

Exposed Aggregate ; The exposed aggregate finish is obtained by

applying a finish coat in which the coarse aggregate is usually from

l/S to l/4 inch and upward in size. This should be proportioned in

accordance with architectural featiires' and effects desired. The

coating is applied and, after stiffening, the surface film and finer

aggregate is removed by gently bimshing with a wire brush and then left

to harden and dry but. Next it should be washed with diluted acid and

clean water. Colored aggregates may be Used to secure varied effects.

This treatment ranks high but is difficult to obtain and should be done

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only "by skilled me,chanics. ;‘This- coarse aggregate,^ due to its density,

tends to eliminate "many of the common 'Structural defects.

miITTMMCE OF STUCCO

Cleaning Stucco : . Portland cement 'stucco may he cleaned hy hose

applications of water and hy brushing with either a wire or fiber brush

ahd then' thoroughly rinsing with deaf' cold water . If unusually dirty,

it may be washed with a solution of 1 part muriatic acid to6 or 7 parts

of water brushed on with a' fiber broom' and then thoroughly rinsed with

Cold water to' remove all traces: of acid.- For white cement stucco, use

a sulphuric acid solution since the muriatic acid tends to produce a yellow tinge on white cement.

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Alterations to a' building often necessitate " , repairing 'stucco, 'fhe methods are few,: being limited to the expensive

replacement of ’the' enta're wall or :by patching areas and pointing the

cracks. Large cracks and" deffeet ive spots are chipped back and the ' adjacent stucco thoroughly wette'd before applying the mortar. Efforts

Should be made , If possible , to- use' the. same brand of cement, proportions, etc . , a’s were' used in .'the original work. Should the renovated

portions after drying contrast badly with the original wall, the entire

structure may be gone' over'wi'lh a thin 'cement wash, as described later.

Paint ing: Changing the' color of stucco can be accomplished by

either' cold- -water washes or Oil paints'.

Cold-Water Washes":'"" -'Obi ore'd 'Cement washes may be obtained by using

various proportions bf' gray and white cement and light or dark sand

with’ small amounts of- nJine'fal pigments.' A practical white color is

produced by mixing 1 p'art white Cement, 1 part yellow sand all passing

through a IToo 20 sieve and 5 percent of hydrated lime by weight of the

cement. Ihese maoerials should be thoroughly mixed dry and of such

quantities as will do’ the '-entire job or at least one side of the structure. As needed, '^the diy' -mixed materials should be poured into a

Container full of Clean 'Water and stirred vigorously to a consistency

of stiff oil paint.' In applying, the mixture should be stirred with

the brush from time to time to keep all the material in suspension. In

refilling the container, all the old wash should be cleaned and thrown

away and the same amount of clean dry materials and water used in

preparing ' each batchl''

Just before applying 'the wash, the wall should be thoroughly

wetted, but, as in the application of stucco, there should be ho free

water on'the surface. 'Starting at the:t'op''dnd one end of the wall,

the wash should be 'brushed.'- on with the’"' thinnest possible coat that willcover, After the coating -has' beeii applied, it should be'gently sprinkled

With water for several ''days' to prevent -hapid drying out of the cement

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which otherwise would dust off. To hasten the early set of the cement

and shorten the need of sprinkling, a solutiom made hy dissolving 4

pounds of commercial calcim chloride crystals in 12 gallons of water

should be used to gage the wash.

Oil Faints ; Although not advised, hut as a last resort, stucco

may he painted with oil paint. This should only he undertaken hy

workmen having considerable experience. It is important to have the

surface thorou^ly dry, and even then it may contain some free lime

which is injurious to oil paints. Some authorities recommend that before painting, structures less than a year old he treated with an aqueous solution of 3 to 4 rounds crystallized zinc sulphate to 1 gallon

of water and allowed to dry thoroughly. The surface can then he painted

with either a white lead mixture or mixtures of white lead and zinc oxide.

The priming coat should contain some boiled linseed oil.

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