Although stucco siding is very durable, it can be damaged, and over time it can crumble or crack. The directions given below work well for patching small areas less than 2 square feet. For more extensive damage, the repair is done in layers, as shown on the opposite page.
Tools & Materials 
Caulk gun
Disposable paintbrush
Putty knife
Mason’s trowel
Square-end trowel
Hammer
Whisk broom
Wire brush
Masonry chisel
Aviation snips
Pry bar
Drill with masonry bit
Scratching tool
Metal lath
Metal primer
Stucco patching compound
Bonding adhesive
Denatured alcohol
Metal primer
Stucco mix
Masonry paint
1 1/2" roofing nails
15# building paper
Self-furring metal lath
Masonry caulk
Tint
Metal stop bead
Particle mask or respirator
Protective equipment
Fill thin cracks in stucco walls with masonry caulk. Overfill the crack with caulk and feather until it’s flush with the stucco. Allow the caulk to set, then paint it to match the stucco. Masonry caulk stays semiflexible, preventing further cracking.
HOW TO PATCH SMALL AREAS
Remove loose material from the repair area using a wire brush. Use the brush to clean away rust from any exposed metal lath, then apply a coat of metal primer to the lath.
Apply premixed stucco repair compound to the repair area, slightly overfilling the hole using a putty knife or trowel. Read manufacturer’s directions, as drying times vary.
Smooth the repair with a putty knife or trowel, feathering the edges to blend into the surrounding surface. Use a whisk broom or trowel to duplicate the original texture. Let the patch dry for several days, then touch it up with masonry paint.
HOW TO REPAIR LARGE AREAS
Make a starter hole with a drill and masonry bit, then use a masonry chisel and hammer to chip away stucco in the repair area. Note: Wear safety glasses and a particle mask or respirator when cutting stucco. Cut self-furring metal lath to size and attach it to the sheathing using roofing nails. Overlap pieces by 2". If the patch extends to the base of the wall, attach a metal stop bead at the bottom.
To mix your own stucco, combine three parts sand, two parts Portland cement, and one part masonry cement. Add just enough water so the mixture holds its shape when squeezed (inset). Mix only as much as you can use in 1 hour. Tip: Premixed stucco works well for small jobs, but for large ones, it’s more economical to mix your own.
Apply a 3/8"-thick layer of stucco directly to the metal lath. Push the stucco into the mesh until it fills the gap between the mesh and the sheathing. Score horizontal grooves into the wet surface using a scratching tool. Let the stucco dry for two days, misting it with water every 2 to 4 hours.
Apply a second, smooth layer of stucco. Build up the stucco to within 1/4" of the original surface. Let the patch dry for two days, misting every 2 to 4 hours.
Combine finish coat stucco mix with just enough water for the mixture to hold its shape. Dampen the patch area, then apply the finish coat to match the original surface. Dampen the patch periodically for a week. Let it dry for several more days before painting.