Addendum to discussion about bonding agents

 

 

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!