There are three main types of glues used in the construction industry. Concrete Glues (Poly Vinyl Acetates), high end bonding agents (Ethyl Vinyl Acetates), and Acrylics. I am convinced these products are used interchangeably and incorrectly most of the time. I have seen contractors putting acrylics on the wall to bond mud or stucco. I have seen them pour Concrete glue in the mixer to modify the mix and add strength. These guys have been using these products wrong for so long that they are convinced they are right! I have called these guys on this many times and the reaction is the same: “I have been doing this for 20 years”! My gut is that this actually does work to some degree. The logical conclusion is that these are all glues and would react whether in the mix or on the wall. However, there is a right way to do things. Acrylics are always admixtures. They modify and strengthen the material they are added to. Acrylics allow the cement in brown coat, stucco, thin set, or any other cement based product to cure longer and therefore denser. The closer (denser) the molecules the harder (higher Psi) and also the better water resistance you will get. Acrylic admixtures can react differently depending on the weather. Guys used to tell me they didn’t like using acrylics because they “spike” the mud. That is, make it set-up more quickly. My teacher told me it made the material cure longer and therefore stronger and better. So which is it? I called my guru and he said the answer is both! When the weather is over about 85°F to 90°F acrylics can set the material more quickly. This is why we recommend using PVA or EVA on the wall instead. The other truth about acrylics is that the price difference between them has to do with percentage of solids. Some companies add more water than others. So it holds true here that “you get what you pay for”. Some very inexpensive acrylics probably have solids in the low teens and the good ones like Merlex Acrylex are in the high 20% to low 30%. This can be triple the amount of actual chemical that does the job!
PVA’s and EVA’s are another source of confusion. These always go on the receiving surface. They are used on the wall if there is a “bond issue”. That is, if there is something on the wall that inhibits suction. Suction is what causes subsequent material to bond to the last material. As in stucco to brown coat or stucco over stucco. So if there is a bond-breaker like paint, sealers, oil, dirt, synthetic stucco, or any other material that inhibits bond then glue should be applied to the surface. The two main characteristic differences are PVA’s re-emulsify. This means they can become a liquid again after drying. That can be good if you are coming back later to apply the coating or it can be bad if you apply the coating and it rains and the coating slides off the wall! EVA’s do not re-emulsify and are best to use if there is any chance of rain or moisture within 72 hours. Again these chemicals are cut with water and the percentage of solids can be very low. Be careful of inexpensive glues and best to trust name brands. Contractors seem to love Weld-Crete®. It is a great product and very high in solids. Weld-Crete is a PVA and will re-emulsify. Mighty Bond from Merlex is a high solid PVA and the price is in the mid range. Merlex also makes an EVA called Superhold that is the best bonder on the market. It has a blue tint to it so you can see where it is (or isn’t) on the wall. It is towards the high end in price but is high in solids and is the right chemical to use outside.
Soka University - Laguna Niguel, CA
So what is the answer? Is it best to use acrylic in the mix to insure bond or put an EVA on the wall or both? The answer is both. The acrylic in the mix will stick to the EVA on the wall for a chemical bond in lieu of a suction bond. You can get away with either but the acrylic must be dispersed properly in the mix, and the bonding agent must be on every square inch of the wall to bond. With both materials there is that much greater probability for success.
I hope that this can help clear up the use (mis-use) of bonding agents in the field. Reality is that whomever reading this is NOT the plastering contractor. More likely it is a homeowner that will have the onerous task of telling the stucco guy his job…. This never seems to go well!