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What holds this stuff together ?
A clear understanding of what holds plaster or stucco mortar together
important in why this material works, and why it fails.
The three kinds of bond are:
1. A mechanical key.
An example of a mechanical key is mortar on metal lath. Mortar
squishes through the holes in the lath and sets up, locking the mortar
in place. The mortar that squishes through is called a "key".
example of mechanical
keys is this old
wood lath and plaster
application. The mortar that squishes through the
gaps in the lath forms keys that lock the mortar in place when it sets
2. Chemical bond.
Chemical bond is used in several ways, through paint on bonding agents,
or bonding admixtures. In paint-on bonding agents, such as plaster
weld, the surface of the bonding agent partially dissolves when wet
plaster is applied.
The wet plaster mixes with this dissolved portion, and hardens, forming
a bond. This isn't voodoo, like some may believe.
Another kind of chemical bond is formed using bonding admixtures. These
add adhesive properties to the mortar, sticking the mortar on the
3. Suction bond.
Suction refers to the absorption of water. A example
of a suction bond is laying bricks. The bricks are porous
and absorb water rapidly. It is this suction that absorbs the mortar
into the brick, creating a permanent bond. This suction also holds
plaster together. Too much suction, or not enough suction, are the two
main reasons that plaster, interior plaster or exterior cement plaster
An example of a high suction substrate is
cement block, also known as cinder block or CMU (concrete masonry
units). Blocks absorb water very rapidly, making it desirable to wet
the blocks before applying mortar. The idea is to slow down the
suction, so the mortar is absorbed into the block before the water is
absorbed out of the mortar. The more water that is sucked out of the
mortar before it sets, the weaker the bond.
This it what is referred to as a high suction
surface if you see this on a bag of
plaster or a manual on stucco. The bag of plaster suggests adding more
the mix for a high suction surface. The idea is the same as wetting the
block. The additional sand retains more water in the mix, allowing more
mortar to be absorbed, and less water.
Other examples of a high suction substrate is a
scratch coat on metal lath, where the scratch coat has set and
partially dried. The scratch coat should be wet down thoroughly before
the brown coat, or double up coat is applied.
The same goes for brick or other surfaces before the
applied. The suction is what holds the mortar on, but high suction can
controlled by wetting the surface. or adding more sand to the mix on
the next coat.
a hose to slow down the suction
before the application of
the finish coat. This assures a good bond and
extends the working life of the mortar.
suction will cause the wall to
fail, or the
mortar to pop off. A couple of examples of too much suction are old
soft porous bricks, or an old stucco scratch coat that has set up for
way too long. These surfaces seem like they can't be wet enough. How we
deal with these surfaces is by using a chemical bond. On cement plaster
(stucco) we use an acrylic admixture in the
mortar. We use acrylic mixed half and half with water. The
acrylic retains the
water in the mix to keep the water from being absorbed too quickly, as
well as forms a chemical bond by adhesion. This works on other exterior
surfaces, or interior surfaces that receive cement plaster.
Other examples : Old blocks that have had the mortar stripped
off. The pores are filled with dry mortar that have too much suction,
Other examples of too much suction are gypsum block, old scratch or
brown coats whet the mortar has been torn off for repair, and existing
finish coats (color coats). On interior plaster, we use paint on
bonding agents to kill the suction by sealing the pores and creating a
Plaster Weld. Euco Weld, and USG plaster bonder work well for
this. I have had failure from other brands, which seem too watery.
paint on bonding agents should never be used for outside stucco,
because they will get wet and fail.
White Portland won't bond
to white Portland, contrary to popular
Readers have written to explain they have had their house re color
and the new color coat is flaking off, or falling off. A popular method
re-color coating a house is to sand blast the color coat back off, at
least most of the way before applying the color. This also removes any
loose and spalling areas.
My sandblasting days are over. We use the same 50-50 mix of
acrylic admixture when re color coating existing stucco. If the surface
has a rough texture or has nonuniform suction, such as a wall that has
patches,it may be desirable to apply a bonding coat first.
A bonding coat consists of the same acrylic mixed in grey cement
mortar, or mixed with pure Portland cement (with no sand), and applied
with a stiff brush or a whisk broom. This leaves the surface rough and
the pores open for suction to support the finish coat.
A good example of a low
suction is concrete, both
place or pre-cast. Another example is old terra cotta tile.
The concrete does absorb water, but very little and slowly. Concrete is
porous, but not porous enough for a good bond. Cement mortar
applied directly to concrete will fail. Sometimes it won't fail
immediately, but it generally starts to fail within about six months.
This is why readers have written to me about their stucco falling off,
and why so many of these fake stone applications fail on new houses.
The small amount of suction and few open pores will hold the mortar on,
but not good enough. We have torn off and re-done fairly new stucco
applications that were done this way. Our solution is the same: acrylic
admixtures. There are some old fashioned methods that work well for
this, and some applications where there is too much suction, but I'll
cover these later in this article. If you aren't too bored, please read
Our mix is the same, half and half acrylic with water in the mix.
We put this in the scratch coat for a typical three coat, 3/4"
application, or if the the concrete is real straight, a thin base coat
that is roughed up using a float or a stiff brush,
before applying the finish, or color coat.
scratching) the scratch coat
not only provides a
mechanical key for the brown coat, but increases
the suction by opening the pores and increasing the surface area.
This is a homemade
scarifier I made from an old refrigerator rack.
Increasing suction may be
desirable for ceilings,
but with caution.
For inside plaster on metal lath ceilings, suction can be increased by
the amount of sand in the mix. For example, instead of 20 shovels of
a bag, the amount of sand can be reduced to about 12 shovels of sand to
This allows the ceiling to be brown coated or doubled up the next day.
Less sand and more plaster makes the mortar more porous and increases
the suction. The idea
is to be able to double the ceiling the next day, instead of letting it
set and dry
the normal two days. Once we did this on some ceilings in Alexandria,
and let them set all weekend. The ceilings had so much suction, we had
to soak them with a hose, and work in small sections because the mortar
was taking up too fast. Taking up is when the mortar becomes firm,
before the initial set. It is probably better to just use a standard
mix, and let it set up for two days.
Suction in the scratch coat can be reduced by adding
more sand to the brown coat, but this also should be done with caution.
Too much sand can also weaken the mix. Poor mortar, that is mortar with
too much sand, is also slippery, and a lot of the mortar goes too waste
from dropping it on the ground.
I have to comment on posts I have seen in places
like the wall and ceiling forum and other places on the web. I have
seen posted that a good mix for stucco ceilings is 3:1, that is about
30 shovels, or 6 buckets of sand for a bag. Please don't believe
this. I doubt anyone who says this has ever put mortar on a
ceiling all day. Our standard mix is about 2:1. Much more than this and
most of the mortar will end up on the ground.
Straw Bale: I have never done any straw bale, and it isn't used
around here. What I've heard is that mud plaster is thrown (dashed)
onto the bales and allowed to dry for a week or so. This provides
suction for the brown coat. If this is wrong, please let me know. There
are other methods of straw bale.
Adobe: I haven't done any adobe, and it isn't used in Virginia,
but it used heavily used in third world countries. Stucco will fail
before long because the cement and sand won't bond to the mud adobe.
Lime is used for a link. Soupy lime is dashed on the adobe block and
allowed to harden and dry. The lime is firm and very porous and
provides a good suction bond for stucco.
Bonding mortar to
concrete: This also applies to other low
I have written about this before, which you may find here: Question2002
An old fashioned method I learned from an
old timer and I have used before
is using a Portland cement paste. This method was used in plastering
swimming pools and on concrete years ago, and still works today. A
paste in made using pure Portland cement and water, to a consistency it
can be applied with a brush. First, the concrete is wet by splashing
with water. The paste is then put on
with a brush, leaving the surface rough. This paste will set up
immediately, and start to dry quickly. It is IMPORTANT to put the
scratch coat of mortar before this paste dries. This can be done in two
ways. The paste can be applied to a small area at a time, say about 3
feet by three feet, and then the mortar scratched on. Another method is
to have two people working, one painting on the paste and the other
scratching mortar on right behind him. The mortar is the scarified or
raked. and allowed to set up and partially dry at least overnight.
There is now good suction to support the brown coat.
Peppering: This is
also a good method if have seen being done in
Mexico, and have seen foreigners doing here. First the concrete is wet
down and rich mortar (mortar with less sand) is splattered on the
surface, leaving the splatters as rough as possible. After this sets
firm and partially dries, a scratch coat of mortar can be put on and
scarified (raked, notice I don't say scratched). This scratch
coat can be browned after it has set up and dried. I have seen concrete
soffits done like this that were three stories high. Just think that
someone could be killed if a piece of this stucco mortar fell off and
hit them on the head. These ceilings will not ever fail if this was
done using this method.
Peppering is also used for using old welded wire or woven wire
lath, where there is no paper behind the lath to support the scratch
coat until it sets up. The holes on the lath are too big or the wire
can't hold the mortar on. First, the metal lath is peppered by
soupy mortar on. Better suction is created by using rich mortar, or
less sand. After this peppering sets and partially dries, suction is
created for better support on the lath, and the lath can then be filled
in with a scratch coat. I have never pumped mortar on a ceiling, but I
have heard that peppering is used before pumping the scratch coat on a
metal lath ceiling. This suction prevents "drop outs" when the
scratch coat is pumped on.
I mentioned I would bring up plastering on old bricks without using chemical bonders.
There is no real good method, except first soaking
the bricks at least twice before beginning. One person can then splash
water on the wall while another immediately applies mortar. If this
mortar starts to dry before it sets, more mortar should be troweled
over the drying mortar before it dries. The idea is prevent the mortar
from drying before it sets, avoiding "dry-outs". These "dry outs" will
leave a soft powdery surface unsuitable for applying more mortar. It is
good to use a lot of sand in the scratch coat for old bricks (meaning
1800's bricks). , say as much as 30 shovels per 94 pound bag of
Portland. The additional sand retains water in the mix longer to
counteract the suction.
No matter what, on old soft bricks, the scratch coat will have tons of
road map shrinkage cracks due to the high amount of suction. These will
fill in with the brown coat and should be of no concern unless the
edges of the cracks curl up, indicating failure to bond. There is
nothing else to do in that case but tear it off and start over.
More about suction some other time. Suction is also used in the brown
and finish coats to provide firmness and and workability.
I want to devote a following article to chemical bonding. Use of
chemicals has improved the quality of our product dramatically, but
be done with caution. Please check back