The materials needed for portland cement plaster, often called "Stucco" are plastic cement, sand and water. If regular cement is used, hydrated lime should be purchased also to make the plaster workable. No lime is to be used with plastic cement because it is complete as formulated.
Plastic cement is a specially formulated type of cement for use in plaster and sometimes masonry. Plastic cement contains all the necessary ingredients, other than sand and water, to make a suitable mixture for troweling onto walls and ceilings. Most plasterers prefer straight plastic cement to avoid possible confusion at the mixer for the ease of application. Some plasterers use a mixture of equal parts of plastic and regular cement. Either formulation is msuitable. A few plasterers still use regular cement to which Flintkote Type S lime is added in the mixer. Unless hydrated lime is added to regular cement, the resulting plaster may be difficult to spread and will not cling well to lath on walls and soffits. No lime should be added to a half-and-half mix of plastic and regular cement.
Sand: Three general classifications of sand, by gradation, normally are available from a dry material yard. They are concrete, sand, plaster sand and masonry sand. Concrete sand, being fairly coarse, would make a good quality plaster wall but is difficult to spread because of coarseness. Masonry sand is too fine for use in the two base coats of portland cement plaster because it may induce cracks in the plaster. A typical plaster sand used in the San Francisco Bay Area is called Oly #2, and is mined in the Felton area of the Santa Cruz Mountains. Oly #2. has a gradation that contributes to ease of spreading, yet is not so fine that it will induce cracking. Oly #1 is masonry sand, not to be used in the scratch and brown coats of cement plaster.
Plaster sand should be clean as well as properly graded. When sand is delivered to the job site, it should be protected against contamination from cigarette butts, scraps of gypsum board, dirt, oil and grease, paper or other harmful contaminants. One may use a cubic foot box to measure sand into the mixer or be guided by shovel count. Normally mixer operators use shovel count and add between twenty and thirty shovels of sand per bag of plastic cement or to the equivalent of a bag of half-and-half plastic and regular, or to a bag of regular plus lime.
Any water supply suitable for human consumption is satisfactory for mixing cement plaster. The most important factor is that one NEVER SHOULD ADD AN EXCESSIVE AMOUNT OF MIX WATER. Use only enough water to produce a spreadable mix. Any additional water will weaken the finished plaster unnecessarily and may induce cracks.
Lime: If one elects to use regular cement, bags of hydrated lime must be purchased at the same time as the other materials. When adding lime to regular cement in the mixer, it is customary to use about fifteen pounds of lime for each 94 pound bag of regular cement. The plasterer should be guided by Table 47-F of the Uniform Building Code in this regard.
Lime is not needed to make plastic cement more spreadable.
Mixing: The minimum volume of plaster sand per bag of cement is three cubic feet. Note that a bag of cement contains one cubic foot of material. The Uniform Building Code (UBC) has established a formula for the first (scratch) and second (brown) coats of cement plaster. That data is found in Table 47-F of the UBC. The maximum ratio of sand to cement in the first coat is four cubic feet of sand for each cubic foot or bag of cement. In the second or brown coat, a maximum of five cubic feet of plaster sand is permitted for each bag of cement.
The preferred sequence of adding ingredients to the mixer is to pour some water into the mixer, followed by part of the sand. Add the necessary amount of cement, one or two bags, depending upon the size of the mixer. Add more water as required, then the balance of the prescribed amount of sand, with additional water as needed to make a workable mix. Be very careful about adding the last bit of water because it is possible to add too much liquid and make a sloppy mix. The danger of adding excessive water is that it would lead to possible weakness in the plaster wall, plus formation of cracks or checking in the complete wall.
Allow the plaster to mix in the machine four or five minutes to gain full benefit from the air entraining agent interground with the plastic cement in the factory. Entrained air is beneficial in improving workability of plaster and reducing the water:cement ratio.
Finish Coat (often called the color coat):
The finish coat, which is the third coat of plaster normally applied by plasterers, is manufactured stucco. Stucco contains all the necessary ingredients to achieve the desired color and texture except for the addition of water at the job site. As is true of the two base coats, only enough water to achieve workability should be added; otherwise pigments may migrate, and the result will be a mottled wall.
Another problem related to excessive water in the finish coat is formation of fine check cracking.
One should be very careful when mixing colored stucco that the same amount of mix water always is added to each batch of stucco, to prevent color variation. It is important that the mixer and working tools be kept clean and free of contaminating dirt, grease or other offensive materials.
The scratch coat should be the thickest of the three coats. If the amount of mix water is controlled properly, one can lay on a half-inch of scratch coat successfully. The first coat must be applied with sufficient material and pressure to solidly fill all openings in the lath. Allow the scratch coat to take up slightly before scoring the plaster lightly in the horizontal direction only. The score marks in the first coat should be shallow.
The brown coat is considered a dressing coat to help level the wall to good alignment, as well as to build the base plaster out to a thickness of three-quarters of an inch before allowing the brown coat to moist cure and then dry in preparation to receive the color coat.
The brown coat should be brought out to proper thickness; troweled, rodded and floated to a suitable surface, straight yet sufficiently rough to provide adequate bond for the finish coat. Compaction with a shingle float is beneficial. UBC requires that the brown coat be in place seven days before the finish coat is applied.
The plasterer will decorate the surface of the applied color coat with a textured finish, which may be a dash texture, sand float finish, mission texture or other attractive surface decoration. Cement plaster should not be troweled smooth, nor should it be overworked with the tools.
Minimum thickness of all three coats must be seven-eighths inch over frame construction.
Curing portland cement plaster is a matter of retaining enough water within the thickness of plaster to chemically hydrate the cement paste that provides strength and other attributes of plaster. Periodic moistening of the plaster is beneficial to keep water in the plaster membrane. Moistening should be done during the cooler periods of the day; in the morning, late afternoon and evening. If the wall is intensely hot, thermal shock with possible attendant cracking may occur when cold water strikes a hot wall.
Replacement of moisture in the wall by hosing is needed during hot weather, windy weather and during very cold weather. Moistening is necessary during very cold weather because cold air frequently is very dry air, and has the capability of drawing moisture out of the plaster at a rapid rate.