Bonding agent failure

Ask the pros- May, 2002
By Reggie Bullard



                     More about bonding agents

I am having my 1969 single story So. Calif. house restuccod and the Sub wants to use a cement type glue to glue (Crete-Weld) over the old painted stucco. I have been told that sandblasting the old stucco is vital for a good bond, he feels that he can get away without sandblasting the old stucco and just applying the glue product directly over the old painted stucco. Can you please comment on this issue?

I got sued when a house we did this way  popped off 4 years later. Jobs we did 10 
years ago are now popping off.  Weld crete dissolves in water. Stucco 
isn't waterproof, and is porous. Water will eventually dissolve the weld crete. 
It may last longer in Southern California due to the lack of humidity, but I doubt much longer. 
I just redid a patch (for free)  I did on a friend's house 9 years ago that was popping off.  The way weld-crete, plaster weld and euco weld work, the surface dissolves 
with the wet plaster (or cement mortar). After the mortar sets, it all dries together 
forming a chemical bond. This works fine for indoor plaster with a warning 
on the can not to be used in wet areas. Trust me here. It has cost me a lot of 
money and embarrasment. 
Weld-crete, plaster weld and euco weld say right on the bucket not intended for
wet areas such as showers. The outside of a house gets wet, too. These bonding
agents work fine for interior plastering, but fail when used for exterior stucco.
We have torn off loose stucco on buildings done this way. It makes me sick to
ride by buildings we did 10-15 years ago that have loose stucco and maybe some
band-aid patches. I didn't know any better.
Acrylic bonding agents work great for stucco on unpainted porous surfaces
such as concrete, old stucco (unpainted), etc. They are used in swimming
pools. The directions on the bucket says mix with mortar and not paint on first.
The acrylic mortar mix can be troweled on or even painted on with a brush, and
left rough for the application of a brown coat or just a finish coat.Examples of these are Acryl60, Flexcon or Milestone.
I worked with an old plasterer years ago who showed me how to bond cement
mortar to concrete using a Portland cement paste. Pure portand(no sand) is
mixed with water and painted on. First the concrete is wet down. Second,
the cement paste is painted on leaving the surface rough. The paste sets instantly
when it hits the concrete, and then starts to dry. A scratch coat of mortar must
be troweled on before it dries. One plasterer can paint a small section at a time,
and then scratch mortar on right away, or one can go ahead painting and the other
scratching(applying mortar.
This method was used in swimming pools years ago, and is still used in foreign
countries. I saw this done when I was in Mexico. Thick stucco ceilings were applied
to concrete slabs with no failure. If they trust it on a ceiling, I trust it, too.

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