The more I learn about the uses of glues the more I am
finding how tricky they can be. We discussed in the last installment of the
proper uses of different types of glue. Some glues are strictly admixtures and
some are particularly suited for the receiving surface. Problems typically
arise with respect to weather and moisture retention. We discussed last time
that acrylic admixtures should slow the set time down thereby making the cement
material denser. This is not the case when it is hot. However, in temperatures
over about 85 degrees F we find that the modified material spikes with the
introduction of acrylic. That is, the material will set the mix more quickly
when it is hot. With respect to PVA and EVA surface preparations we are finding
some of the same inconsistencies. As plasterers and support personnel, we
constantly look for new ways to improve the outcome of our products and finishes.
I was recently on a job in Santa Barbara where the plasterer was re-scrubbing
an existing Santa Barbara Finish (smooth) parameter wall. Since a smooth coat
was already on the wall (unpainted) the plastering contractor felt it prudent
to glue the wall before scratching in the first coat of smooth stucco. As it
was a warm sunny day the glue appeared dry and the scratch coat was applied. As
the crew doubled back over blisters began to appear. All the discussions that I
have had about blisters are there is too much moisture in the material and the
second coat breaks the set and bond of the first coat. What I have found since
is that judging the readiness of the first coat when there is glue on the wall
is tricky. It appears dry but in reality the glue is holding moisture on the
inside of the first coat. Some plasterers always wait over night following the
application of glue to make sure it is completely dry. These bonders all
re-emulsify as the wet mud is introduced over the top. The difference is that a
PVA will continue to re- emulsify for up to a week (or more depending on
weather) and an EVA will only re-emulsify for 72 hours. The distinction is important
and, at the same time, very tricky. The weight of the stucco can pull itself
off the wall if the glue re-emulsifies following application. So we want the
glue to “wet up” as we apply the stucco to insure a chemical bond, but we want
it to cure before an appreciable amount of moisture is introduced. Another
problem is when a bonding agent is put on the wall it will again hold moisture.
This time it is after both coats of smooth (Santa Barbara Finish) is applied
and the material has had its final “hard trowling”. The material looks great
but since there is moisture present it continues to cure over night and the
moisture naturally wicks out and hydrates. This leaves “checking” in these
areas. This is also known as “spider web cracking,” “crazing,” or “shrinkage
cracks.” Some of these are considered normal and part of the finish as it is
virtually impossible to eliminate these small cracks.
Consider also that if there is suction (as with the aforementioned
re-stucco of an existing smooth wall) should we use glue? In this case we are eliminating
the suction bond in lieu of a chemical bond. I have always been partial to a
suction bond whenever we can get it. As I interviewed more mechanics about this
issue I noticed that some have found (receiving surface type) bonding agents to
be tricky enough to quit using them. These guys have gone to using a polymer
modified base coat similar to Basex to base out the wall. This is a way to introduce
a polymer modified cement coating to bond to the existing wall and still have
suction and key to bond the finish coat.
When I discuss polymer modified cements, base coats and
glues, I usually find myself voicing one of my favorite adages: “for every
solution comes its own set of problems; we just hope the solution is better
than the problems they bring!” We are
all “big boys” when it comes to plastering and construction and know that we
picked a trade that is very difficult. We get blamed for every crack and
imperfection in the finish coat and sometimes have no answers why. As we try to
elevate the industry by using new products and techniques we have to remind
ourselves that it is worth the effort. At least we are out there trying. Who
knows? Maybe we will find a cementitious finish that someday will have all the
functionality we want with none of the present problems. Until then, let’s just
keep thinking!
