Stucco FAQ according toAmerican Cement Association

Not directly. Paint keeps the brick from absorbing water, so stucco cannot bond to it evenly. There are two ways to do it:

• Strip the paint completely by sandblasting or water blasting, so the whole surface absorbs evenly, then apply a two-coat system straight onto the brick, often with a bonding agent or a dash bond coat.
• Leave the paint, hang building paper and metal lath over the wall (or use paper-backed lath), and apply three-coat stucco to the lath. The paper keeps the stucco from bonding to the wall in patches, which would set up stresses and cause cracks.

Yes. Like every cement-based material, stucco needs moisture and a moderate temperature in its first days to harden properly. Its coats are thin, from 3/8 to 7/8 inch in all and sometimes 1/8 inch each, so sun and wind can dry them out fast.

Base coats are misted once or twice a day once they are firm enough not to wash off, usually for the first three to seven days. The first two days matter most. Plastic sheeting can hold the moisture in, and screens against sun and wind help in harsh weather. When the humidity is above 70%, extra wetting may not be needed.

Colored finish coats are not sprayed, because water can leave them blotchy; instead the base coat is wetted so moisture reaches the finish from behind. The usual working range is about 40°F to 90°F. Colder risks freezing, and hotter dries the stucco too fast.

Not the spacing. Control, contraction and expansion joints all exist to relieve stress, and the spacing rules (see the joint spacing question below) apply to all of them. Spacing matters most on framed walls. On block or concrete, joints simply follow the joints in the wall behind the stucco.

What changes is the accessory. One-piece joints with a pleated or accordion shape handle movement in the plane of the wall. Where the wall can move in more than one direction, such as where the backing changes, around doors and windows, and where a wall meets a roof or a second story, a two-piece joint or back-to-back casing beads work better.

Treat the foam face like sheathing: building paper first, then metal lath fastened to the furring strips built into many ICFs, then three-coat stucco. The paper separates the stucco from the foam, and the lath carries its weight. Stucco would stick to the foam, but the foam cannot hold that weight, so the stucco could sag or shear off. ASTM C1063 covers the lath, fasteners and joint spacing, and ASTM C926 does not cover stucco applied straight to ICFs.

Usually not. On framed walls the stucco goes over paper and lath, so there is nothing for it to bond to and bonding agents are not used. They are a tool for solid surfaces, concrete or block, that will not bond well: older walls with paint, sealer or dirt on them, or new poured concrete with form-release oil on its face.

The better first step is to make the surface clean, sound and rough, by sandblasting or high-pressure water. If that is not enough, a bonding agent can help (surface-applied products follow ASTM C932), or a dash bond coat: a cement-rich slurry thrown onto the wall by brush, trowel or machine and left rough for the scratch coat.

Yes. Building codes set minimum waiting times between exterior coats, so each coat can cure and gain strength before the next one goes on. The exception is the double-back method: the brown coat can go onto the scratch coat as soon as the scratch coat is firm enough not to be damaged, often the same day, and the new coat then holds the moisture in. Following the timing rules gives the most even color and texture.

It depends on what is behind it. The nominal thicknesses in ASTM C926:

• Three coats over metal lath (framed walls, or lath over block): 7/8 inch
• Three coats on a solid surface without lath: 5/8 inch
• Two coats straight on concrete block or brick: ½ inch
• Two coats straight on poured or precast concrete: 3/8 inch

They are nominal because walls are never perfectly flat or plumb. Your local building department can tell you about any variations it allows.

About 10.4 pounds per square foot for three-coat stucco, 7/8 inch thick over metal lath. The mix itself weighs about 142 pounds per cubic foot, close to mortar. Block and concrete walls carry that easily, and new framing is normally designed for it, but when stucco goes onto an older framed building, the framing should be checked first.

On block or concrete with no lath, joints are needed only where the wall material changes or where the wall itself has joints. Wherever metal lath is used, ASTM C1063 limits the size of each panel:

• No more than 18 feet long in either direction
• No more than 144 square feet on walls
• No more than 100 square feet on ceilings and curved or angled surfaces
• No more than 2½ times as long as it is wide

To keep water off the sheathing and framing behind the stucco. Wood swells and shrinks with moisture and can rot or grow mold, metal can rust, and dry sheathing gives the stucco a steady base. Industry guides and building codes call for two layers, because paper can be torn during construction, and the layers are lapped like siding, upper over lower, so water drains out.

Grade D building paper is the usual choice. House wraps are tougher, but one layer of house wrap alone is generally not considered enough; a common alternative is house wrap against the sheathing with paper over it. Over ICFs, the paper works mainly as a bond breaker, letting the stucco and the foam move independently.

Yes. Cement-based paints suit stucco best because they are made of the same material; they are scrubbed into the surface and left to cure. Another option is a colored finish coat, made with white cement and pigment, which offers the widest range of colors and, at about 1/8 inch thick, covers more completely than paint. Other paints work too but usually do not last as long, and acrylic paints, though durable, make the wall less breathable, which can matter in some climates.

A thin cement slurry, the same cement, lime and water as the original finish but without the sand, sprayed or rolled on to even out the color of a stucco wall. It freshens the look without changing how the stucco lets water vapor pass through.

Cornices, quoins and similar trim are usually "plant-ons": shapes cut from expanded polystyrene foam, glued to the stucco base coats with an adhesive made for the purpose, then covered with base coat and mesh and finished like EIFS. The foam has no strength of its own, so the mesh and base coat are what make the shape tough. The Northwest Wall and Ceiling Bureau's Stucco Resource Guide has sample details.

Yes. The right method depends on the stain. For everyday dirt, wet the wall first, from the bottom up, so dirty water is not drawn into the dry pores, then rinse with a garden hose or a fan-tip sprayer. Warm water, a brush or a mild detergent add cleaning power. Go easy with pressure washers, since water under high pressure can etch or even cut hardened stucco, and keep the spray moving. Test any method on a hidden spot first.

The Technical Service Information Bureau (TSIB), a wall and ceiling industry group in Southern California, shows about 30 plaster textures online, with notes on how each is made, from light, medium and heavy sand finishes to troweled textures. Names for the same finish vary by region, which is why pictures help.