Exterior stucco consists of a layer of cement troweled (in some cases sprayed) on over a backing of wire lath (metal mesh), or over a backing of con- [TEXT OMITTED FROM SOURCE] The stucco may be relatively smooth and flat, or it may have a highly textured finish.
When cracks or breaks appear in a stucco surface they should be repaired as soon as possible -- ideally when the weather is well above freezing but not yet up into the 80s. Prompt patching will keep additional water from making matters worse by rusting the wire lath, rotting wood framing and structural members, and causing deterioration of the stucco coating around the cracked or broken area.
Repairs ae usually quite simple to make when cracks are first noticed. Start by using a stiff putty knife or old chisel to undercut the edges of the crack along each side, so the crack is actually wider on the inside than it is at the surface. If any material feels loose, chip it away so what remains is solidly bonded to the backing or base underneath. Use a wire brush to scrub out loose, powdery material, then wet the crack uniformly by brushing or spraying on water.
To fill the crack you can use ready-mixed dry patching cement (cement and sand mixture) that is sold in all hardware stores, lumberyards and home centers. To mix your own, use one part Portland cement to three parts fine sand. Either way, add enough water to make a reasonably stiff but workable mixture. Pack this tightly into the crack and smooth it off level with the surface. Let it dry until it starts to stiffen (about 15 to 20 minutes), then texture the surface to match the surrounding area. To help it cure properly, keep this patch moist for about 48 hours by spraying with a light mist several times a day.
If the crack is over 1 inch deep, and more than 1/4 inch to 1/2 inch wide, it is best to fill it in two layers. Pack the first layer in firmly, filling about two-thirds the way to the surface (leave at least 1/4 inch to fill with the second layer). Allow to dry about 24 hours, then dampen again and apply the second layer to bring it up flush, and finish opff as described above.
Sometimes as you start to undercut a crack you will find the stucco is loose and bulged outward on each side. If this is so, there is no sence merely patching opr filling the crack; all loose material should be chipped or pried off and a complete new layer of stucco added.
This is also true when you discover a whole section bulging outward and starting to fall away from the wall behind it. All this loose material must be chipped away down to the lath if necessary, and new material applied.
To remove the loose or bulging material use a cold chisel and a hammer. Work carefully and use light blows to avoid loosening any more of the old stucco, but keep chipping until you have removed all loose material down to the lath (or cement block) behind it.
On wood frame houses you will find the wire lath has been nailed on over tar paper with special nails or spacers that keep the lath or mesh about 1/4 inch away from the surface under it.
After all loose stucco has been removed, use a stiff brush to clean out dirt, dust and powdery material, then examine the exposed wire lath. If it is still solid and in relatively good condition you can apply new material directly over it. If it is rusted and starting to deteriorate, cut out the damaged section with tin snips and cut a new piece of wire mesh to fit in its place. If the tar paper underneath is torn, this too should be covered with the same kind of paper or felt.
When nailing the new wire lath or mesh in place space it so it remains about 1/4 inch away from the surface behind it, not flat against it. There are special nails to accomplish this, or you can use washers or nuts around each nail to act as spacers. (The spacers should be behind the mesh.) Be sure the nails and any hardware used as spacers are galvanized.
Large patches of stucco are usually filled in two or three applications -- two if the patch is not more than 1 inch deep, three if the patch is deeper. The first coat, known as the scratch coat, can be mixed by adding 3 parts fine sand to 1 part cement, or you can patch with ready-mixed dry cement and sand.
Add enough water to make a mix that will hold its shape without sagging when mounded, yet will not crumble when troweled. Use a plasterer's steel trowel to spread this over the wire lath, pressing it hard against the surface to force the mix through the mesh.
It is usually easier to work from the bottom up, but this is not critical -- just put enough on to completely embed the wire lath and build up to about a 1/2-inch-thick layer. Allow this scratch coat to stiffen slightly (about 20 minutes), then scratch its entire surface. You can use a small piece of wire lath or other mesh for scratching, or you can drive a row of nails into a piece of wood and rake this across the surface.
Keep the surface damp and allow it to cure for about 24 hours, then apply the second, or brown, coat. Although the same cement-and-sand mix can be used for the brown coat, it works better and tends to bond better if you use mortar mix, which also comes in ready-mixed dry form.
If you wish, you can accomplish the same thing by mixing three parts sand with mortar cement (instead of Portland cement.).
This coat is troweled on over the scratch coat with a steel trowel to a depth of about 3/8 inch, after first dampening the scratch coat. It should be thick enough to completely cover the scratch coat and come to within about 1/8 inch of filling the area completely flush.
The surface is then rubbed with a wood float (a trowel with a wood face), or with a piece of plywood that has a small block of wood on one side to act as a handle.
The brown coat is kept damp while curing for the first two days, then it is allowed to harden for another three to five days before the final coat is applied. The final coat or mortar mix is troweled on after dampening the surface of the brown coat, then it is textured to match the surface of the rest of the stucco.
In some cases this texturing can be done with a trowel -- swirling it back and forth and lightly gouging the surface as the cement is troweled on. In other cases the correct texture can be achieved by stroking with a small whisk broom, throwing pebbles against the wet cement, working a wood float over the surface with a circular motion, rubbing with burlap, or simply patting with a steel trowel as the cement starts to stiffen. Any method that will closely match the original texture is suitable.