A.Ron Webber, a veteran
plastering contractor in Orange, Calif.,
responds: I do not recommend using plastic
stucco cement — actually a blend of
portland cement, silica sand, lime, and fillers to
increase plasticity — for patching. It has
a high water content and develops a lot of very
small shrinkage cracks as it dries, which can cause
a patch to break apart prematurely. A better
approach is to repair dead hairline cracks
— where the stucco is stable and the
cracks don’t change size or reappear
— by dusting them with premixed, pigmented
stucco. I pour dry stucco powder into a cup, then
use a dry 1-inch brush to dab the powder into the
cracks. Immediately after dusting, I brush any
excess off the edges (to prevent buildup) and blend
in the patch. Moisture from the night air will help
the cement set up overnight.
This technique is quick and easy and gives me a
better color match than a wet-mortar repair.
Dusting allows me to blend the patch in with the
existing texture; simply filling in the cracks
tends to leave a telltale “snail
trail” of different stucco textures.
If the cracks are live and reappearing, you will
have to take more extreme measures. For larger
cracks, I mix up a stucco fog coat (pigmented
stucco without the sand) and inject it into the
crack with a large syringe. (I moisten the area
around the crack with water beforehand.) I use a
small chip brush as needed to brush away the excess
buildup and mist the patch as it dries; this helps
harden the cement and makes the mineral pigment
more stable.
If the cracks are too small, though, the fog
coat won’t penetrate them very well and
the patch won’t be as strong. To avoid
this, you can dig out the cracks, moisten the area
around them, and then squeeze the wet stucco into
the cracks. It helps to brush some diluted acrylic
admix into the cracks to help with adhesion. You
might need to scrape back some of the finish coat
and retexture the stucco, but matching an existing
texture is a repair that requires good hands.
Again, misting water onto the patch as it dries
helps a lot.
While I’ve seen other plasterers fill
cracks with urethane caulking, it’s not a
method I use. Pigmented acrylics and urethanes
might closely match the stucco when they are dry,
but when it rains and the walls are wet, every
patch will show up. This is because portland cement
stucco absorbs water and darkens, while acrylics
and urethanes don’t. Of course, if the
walls are painted, then the color difference
won’t be as obvious when it rains.